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.h b/src/compositor.h
index c39abe3..fdde762 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -738,7 +738,7 @@
 weston_seat_init(struct weston_seat *seat, struct weston_compositor *ec);
 void
 weston_seat_init_pointer(struct weston_seat *seat);
-void
+int
 weston_seat_init_keyboard(struct weston_seat *seat, struct xkb_keymap *keymap);
 void
 weston_seat_init_touch(struct weston_seat *seat);