compositor: destroy all input binding objects on shutdown

wlsc_binding_destroy() was not being called at all. Fix the leaks by
introducing a function that destroys a whole list of bindings, instead
of individually saving an extra pointer to the binding object and
calling wlsc_binding_destroy() separately on each.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/compositor/compositor.c b/compositor/compositor.c
index c4ab9db..980098d 100644
--- a/compositor/compositor.c
+++ b/compositor/compositor.c
@@ -2022,6 +2022,8 @@
 	/* Destroy all outputs associated with this compositor */
 	wl_list_for_each_safe(output, next, &ec->output_list, link)
 		output->destroy(output);
+
+	wlsc_binding_list_destroy_all(&ec->binding_list);
 }
 
 static int on_term_signal(int signal_number, void *data)