x11-toolkits/gtk-sharp20: fix build with clang 15

During an exp-run for llvm 15 (see bug 265425), it turned out that
x11-toolkits/gtk-sharp20 failed to build with clang 15:

  windowmanager.c:113:12: error: incompatible integer to pointer conversion assigning to 'gpointer' (aka 'void *') from 'long' [-Wint-conversion]
                  list [i] = data [i];
                           ^ ~~~~~~~~

This is because gdk_property_get() always returns an array of longs, and
these have to be cast to whatever the desired outcome type is.

PR:		268812
Approved by:	portmgr (tcberner)
MFH:		2023Q1

x11-toolkits/gtk-sharp20/files/patch-gdk_glue_windowmanager.c

@@ -0,0 +1,11 @@
+--- gdk/glue/windowmanager.c.orig	2016-09-22 10:17:18 UTC
++++ gdk/glue/windowmanager.c
+@@ -110,7 +110,7 @@ gtksharp_get_gdk_net_client_list (int *count)
+ 	list = g_malloc (*count * sizeof (gpointer));
+ 	/* Put all of the windows into a GList to return */
+ 	for (i = 0; i < *count; i ++) {
+-		list [i] = data [i];
++		list [i] = (gpointer) data [i];
+ 		g_message ("WinID: %d", list [i]);
+ 	}
+