Install bash completion files in /usr/share/

The bash-completion code nowadays expects completion files to
be installed in  /usr/share/bash-completion/completions, and
expects them to be named like the command they are completing
for.

https://bugzilla.gnome.org/show_bug.cgi?id=677782

gio/Makefile.am

@@ -721,11 +721,11 @@ gdbus_LDADD = libgio-2.0.la \
 	$(top_builddir)/glib/libglib-2.0.la 		\
 	$(top_builddir)/gobject/libgobject-2.0.la
 
-completiondir = $(sysconfdir)/bash_completion.d
+completiondir = $(datadir)/bash-completion/completions
 completion_DATA = \
-	gdbus-bash-completion.sh		\
-	gsettings-bash-completion.sh		\
-	gresource-bash-completion.sh
+	completion/gdbus				\
+	completion/gsettings				\
+	completion/gresource
 EXTRA_DIST += $(completion_DATA)
 
 # ------------------------------------------------------------------------