Terminate on Super-Backspace.

It's almost like Ctrl-Alt-Backspace!
diff --git a/wayland-system-compositor.c b/wayland-system-compositor.c
index c135aea..dce6d2f 100644
--- a/wayland-system-compositor.c
+++ b/wayland-system-compositor.c
@@ -1193,7 +1193,7 @@
 		return;
 
 	switch (key | compositor->meta_state) {
-	case KEY_EJECTCD | META_DOWN:
+	case KEY_BACKSPACE | META_DOWN:
 		on_term_signal(SIGTERM, compositor);
 		return;