compositor: Add kill signal

For now it's being emitted only on the desktop shell kill binding.

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>
diff --git a/src/compositor.h b/src/compositor.h
index aea4f8d..bde4772 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -294,6 +294,7 @@
 	struct weston_shell_interface shell_interface;
 
 	struct wl_signal activate_signal;
+	struct wl_signal kill_signal;
 	struct wl_signal lock_signal;
 	struct wl_signal unlock_signal;