meson: Set install_tag on systemtap files

This could be done automatically by Meson, this commit can be reverted
when we have that Meson PR in our CI:
https://github.com/mesonbuild/meson/pull/10829

glib/meson.build

@@ -488,6 +488,7 @@ if enable_systemtap
     output : '@0@.stp'.format(libglib.full_path().split('/').get(-1)),
     configuration : stp_cdata,
     install_dir : tapset_install_dir,
+    install_tag : 'systemtap',
   )
 endif
 

gobject/meson.build

@@ -179,6 +179,7 @@ if enable_systemtap
     output : '@0@.stp'.format(libgobject.full_path().split('/').get(-1)),
     configuration : stp_cdata,
     install_dir : tapset_install_dir,
+    install_tag : 'systemtap',
   )
 endif