patch 8.1.0941: macros for MS-Windows are inconsistent

Problem:    Macros for MS-Windows are inconsistent, using "32", "3264 and
            others.
Solution:   Use MSWIN for all MS-Windows builds.  Use FEAT_GUI_MSWIN for the
            GUI build. (Hirohito Higashi, closes #3932)
diff --git a/src/gui_gtk.c b/src/gui_gtk.c
index 3d08934..ba8fd9d 100644
--- a/src/gui_gtk.c
+++ b/src/gui_gtk.c
@@ -76,7 +76,7 @@
 #  include <gdk/gdkkeysyms.h>
 # endif
 # include <gdk/gdk.h>
-# ifdef WIN3264
+# ifdef MSWIN
 #  include <gdk/gdkwin32.h>
 # else
 #  include <gdk/gdkx.h>