examples: fix exit from "vpn-import-libnm.c" example

../examples/C/glib/vpn-import-libnm.c: In function main:
  ../examples/C/glib/vpn-import-libnm.c:72:1: error: control reaches end of non-void function [-Werror=return-type]
   }
   ^

Fixes: 905f9975d2 ('example: importing vpn with libnm')

examples/C/glib/vpn-import-libnm.c

@@ -69,4 +69,6 @@ main(int argc, char **argv)
 
     nm_client_add_connection_async(client, conn, TRUE, NULL, (GAsyncReadyCallback) add_cb, loop);
     g_main_loop_run(loop);
+
+    return 0;
 }