xkb: Don't call exit on failure in weston_compositor_xkb_init()

This will exit without cleaning vt modes and leave the system stuck.

https://bugs.freedesktop.org/show_bug.cgi?id=60817
diff --git a/src/compositor-x11.c b/src/compositor-x11.c
index e39c482..7a9998c 100644
--- a/src/compositor-x11.c
+++ b/src/compositor-x11.c
@@ -303,7 +303,8 @@
 	weston_seat_init_pointer(&c->core_seat);
 
 	keymap = x11_compositor_get_keymap(c);
-	weston_seat_init_keyboard(&c->core_seat, keymap);
+	if (weston_seat_init_keyboard(&c->core_seat, keymap) < 0)
+		return -1;
 	if (keymap)
 		xkb_map_unref(keymap);