gl-renderer.c: Cast wl_resource to void * to avoid warning

The extension has been updated to take a wl_resource * now, but to avoid
warnings when compiling against older versions, just cast to void *.

src/gl-renderer.c

@@ -1370,8 +1370,7 @@ gl_renderer_attach(struct weston_surface *es, struct weston_buffer *buffer)
 
 	if (shm_buffer)
 		gl_renderer_attach_shm(es, buffer, shm_buffer);
-	else if (gr->query_buffer(gr->egl_display,
-				  (struct wl_buffer *)buffer->resource,
+	else if (gr->query_buffer(gr->egl_display, (void *) buffer->resource,
 				  EGL_TEXTURE_FORMAT, &format))
 		gl_renderer_attach_egl(es, buffer, format);
 	else {