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_f.c b/src/gui_gtk_f.c
index 3f376c0..d81474b 100644
--- a/src/gui_gtk_f.c
+++ b/src/gui_gtk_f.c
@@ -32,7 +32,7 @@
 #if !GTK_CHECK_VERSION(3,0,0)
 # include <gtk/gtksignal.h>
 #endif
-#ifdef WIN3264
+#ifdef MSWIN
 # include <gdk/gdkwin32.h>
 #else
 # include <gdk/gdkx.h>