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.c b/src/compositor.c
index 417c508..95ee258 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -2737,6 +2737,7 @@
 	ec->wl_display = display;
 	wl_signal_init(&ec->destroy_signal);
 	wl_signal_init(&ec->activate_signal);
+	wl_signal_init(&ec->kill_signal);
 	wl_signal_init(&ec->lock_signal);
 	wl_signal_init(&ec->unlock_signal);
 	wl_signal_init(&ec->show_input_panel_signal);