input: Remove wl_seat global when a seat is destroyed

The input code was relying on compositor destruction to clean up the
global, but that doesn't work when the global comes and goes dynamically.

https://bugs.freedesktop.org/show_bug.cgi?id=65913
diff --git a/src/compositor.h b/src/compositor.h
index 8070409..84f39e2 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -437,6 +437,7 @@
 struct weston_seat {
 	struct wl_list base_resource_list;
 
+	struct wl_global *global;
 	struct weston_pointer *pointer;
 	struct weston_keyboard *keyboard;
 	struct weston_touch *touch;