xwayland/window-manager: Handle theme destruction

Memleak found by ASAN:

Direct leak of 40 byte(s) in 1 object(s) allocated from:
    #0 0x7fe7a917fe8f in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.6+0xa9e8f)
    #1 0x7fe7a5a40736 in theme_create ../shared/cairo-util.c:419
    #2 0x7fe7a5a3363c in weston_wm_create ../xwayland/window-manager.c:2619
    #3 0x7fe7a5a2017e in weston_xwayland_xserver_loaded ../xwayland/launcher.c:313
    #4 0x7fe7a90b4d14 in handle_sigusr1 ../compositor/xwayland.c:57
    #5 0x7fe7a8c2585d in wl_event_source_signal_dispatch ../src/event-loop.c:685
    #6 0x7ffcdb04ef6f  ([stack]+0x1df6f)

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

xwayland/window-manager.c

@@ -2668,6 +2668,7 @@ weston_wm_destroy(struct weston_wm *wm)
 	/* FIXME: Free windows in hash. */
 	hash_table_destroy(wm->window_hash);
 	weston_wm_destroy_cursors(wm);
+	theme_destroy(wm->theme);
 	xcb_disconnect(wm->conn);
 	wl_event_source_remove(wm->source);
 	wl_list_remove(&wm->selection_listener.link);