xdg-shell: Bump unstable version

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/desktop-shell/shell.c b/desktop-shell/shell.c
index a61b27c..bfc6a7b 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -4224,7 +4224,7 @@
 		return 0;
 	}
 
-#define XDG_SERVER_VERSION 4
+#define XDG_SERVER_VERSION 5
 
 	static_assert(XDG_SERVER_VERSION == XDG_SHELL_VERSION_CURRENT,
 		      "shell implementation doesn't match protocol version");