compositor: Add create_surface_signal

This signal is emitted when a client creates a surface, not for internal
surfaces.
diff --git a/src/compositor.h b/src/compositor.h
index 15d6e03..af41b00 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -574,6 +574,7 @@
 	struct weston_config *config;
 
 	/* surface signals */
+	struct wl_signal create_surface_signal;
 	struct wl_signal activate_signal;
 	struct wl_signal transform_signal;