Expose window_set_surface() in window.h

gears.c uses this routine and was complaining about it being implicitly
declared.

Signed-off-by: Bryce Harrington <bryce@canonical.com>

clients/window.h

@@ -157,6 +157,9 @@ window_copy_surface(struct window *window,
 void
 window_flush(struct window *window);
 
+void
+window_set_surface(struct window *window, cairo_surface_t *surface);
+
 void
 window_create_surface(struct window *window);