introspection: Explicitly include libgdk-x11.la when scanning gtk

Debian changed the behavior of libtool to not follow dependencies
from .la files.  Fortunately, the scanner explicitly looks at the
.la files to see if they're really shared; if not it doesn't add
them to the .gir, so there's no cost.

https://bugzilla.gnome.org/show_bug.cgi?id=633405

gtk/Makefile.am

@@ -915,6 +915,9 @@ Gtk_3_0_gir_CFLAGS = \
 		-UGTK_DISABLE_DEPRECATED \
 		-DGTK_TEXT_USE_INTERNAL_UNSUPPORTED_API
 Gtk_3_0_gir_LIBS = $(gtktargetlib)
+if USE_X11
+Gtk_3_0_gir_LIBS += $(top_builddir)/gdk/x11/libgdk-x11.la
+endif
 Gtk_3_0_gir_FILES = $(introspection_files)
 INTROSPECTION_GIRS += Gtk-3.0.gir