patch 8.1.2366: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/gui_gtk_f.h b/src/gui_gtk_f.h
index be7693a..5267c6a 100644
--- a/src/gui_gtk_f.h
+++ b/src/gui_gtk_f.h
@@ -70,10 +70,9 @@
 	gint x, gint y,
 	gint w, gint h);
 
-/* These disable and enable moving and repainting respectively.  If you
- * want to update the layout's offsets but do not want it to repaint
- * itself, you should use these functions.
- */
+// These disable and enable moving and repainting respectively.  If you
+// want to update the layout's offsets but do not want it to repaint
+// itself, you should use these functions.
 
 void gtk_form_freeze(GtkForm *form);
 void gtk_form_thaw(GtkForm *form);
@@ -82,4 +81,4 @@
 #ifdef __cplusplus
 }
 #endif
-#endif	/* __GTK_FORM_H__ */
+#endif	// __GTK_FORM_H__