patch 9.1.0437: Motif requires non-const char pointer for XPM  data

Problem:  Motif requires non-const char pointer for XPM data shared with
          GTK (Tony Mechelynck, after v9.1.0432)
Solution: Cast non-const to const char pointer for GTK (Drew Vogel).

closes: #14834

Signed-off-by: Drew Vogel <dvogel@github>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/gui_gtk_x11.c b/src/gui_gtk_x11.c
index 066aa17..67ee531 100644
--- a/src/gui_gtk_x11.c
+++ b/src/gui_gtk_x11.c
@@ -2727,9 +2727,9 @@
 	 */
 	GList *icons = NULL;
 
-	icons = g_list_prepend(icons, gdk_pixbuf_new_from_xpm_data(vim16x16));
-	icons = g_list_prepend(icons, gdk_pixbuf_new_from_xpm_data(vim32x32));
-	icons = g_list_prepend(icons, gdk_pixbuf_new_from_xpm_data(vim48x48));
+	icons = g_list_prepend(icons, gdk_pixbuf_new_from_xpm_data((const char **)vim16x16));
+	icons = g_list_prepend(icons, gdk_pixbuf_new_from_xpm_data((const char **)vim32x32));
+	icons = g_list_prepend(icons, gdk_pixbuf_new_from_xpm_data((const char **)vim48x48));
 
 	gtk_window_set_icon_list(GTK_WINDOW(gui.mainwin), icons);