Bug 1661450 - 7/8 Special care for mozgtk.c r=stransky

Opt out of X11 code makes mozgtk.c empty. But following the comment in the
file, it requires at least a symbol to force libxul to depend on it.

Therefore I added a common gtk function. However, I don't know if adding that
chosen function can have unwanted side-effects.

Depends on D139532

Differential Revision: https://phabricator.services.mozilla.com/D139533

widget/gtk/mozgtk/mozgtk.c

@@ -6,7 +6,8 @@
 
 #include "mozilla/Types.h"
 
-#include <X11/Xlib.h>
+#ifdef MOZ_X11
+#  include <X11/Xlib.h>
 // Bug 1271100
 // We need to trick system Cairo into not using the XShm extension due to
 // a race condition in it that results in frequent BadAccess errors. Cairo
@@ -19,3 +20,4 @@
 // ever can remove this workaround for system Cairo, we'll need something
 // to replace it for that purpose.
 MOZ_EXPORT Bool XShmQueryExtension(Display* aDisplay) { return False; }
+#endif