Fix the grace build/ld error when build against openSUSE TW.

"$(filter-out drbd_buildtag.o,$(drbd-y))" will translate to
all the .o except drbd_buildtag.o

Error log:
[  110s]   UPD     /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/compat.h
[  110s]   CC [M]  /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_debugfs.o
[  111s]   GEN     /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_buildtag.c 
[  111s]   CC [M]  /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_buildtag.o
[  112s]   LD [M]  /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd.o
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_bitmap.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_proc.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_sender.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_receiver.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_req.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_actlog.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/lru_cache.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_main.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_strings.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_nl.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_interval.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_state.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd-kernel-compat/drbd_wrappers.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_nla.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_transport.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/kref_debug.o: No such file or directory
[  112s] ld: cannot find /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd_kref_debug.o: No such file or directory
[  112s] make[3]: *** [/usr/src/linux-5.0.3-1/scripts/Makefile.build:465: /home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default/drbd.o] Error 1
[  112s] make[2]: *** [/usr/src/linux-5.0.3-1/Makefile:1568: _module_/home/abuild/rpmbuild/BUILD/drbd-9.0.17~1+git.b9abab2d/default] Error 2
[  112s] make[1]: *** [Makefile:152: sub-make] Error 2

diff -Naur drbd-9.0.17+git.b9abab2d.orig/drbd/Kbuild drbd-9.0.17+git.b9abab2d/drbd/Kbuild
--- drbd-9.0.17+git.b9abab2d.orig/drbd/Kbuild	2019-04-01 16:13:18.819447480 +0800
+++ drbd-9.0.17+git.b9abab2d/drbd/Kbuild	2019-04-03 15:13:56.227029581 +0800
@@ -124,7 +124,7 @@
 # and not in e.g. dash. I'm too lazy to fix it to be compatible.
 SHELL=/bin/bash
 
-$(obj)/drbd_buildtag.c: $(filter-out drbd_buildtag.o,$(drbd-y))
+$(obj)/drbd_buildtag.c:
 	@$(kecho) '  GEN     $@ $(echo-why)'
 	@set -e; exec > $@.new;							\
 	echo -e "/* automatically generated. DO NOT EDIT. */";			\