shell_dbus_acquire_name: Don't leak the result

The GVariant returned by g_dbus_proxy_call_sync must be
freed with g_variant_unref, to prevent a leak.

src/main.c

@@ -75,6 +75,7 @@ shell_dbus_acquire_name (GDBusProxy  *bus,
       exit (1);
     }
   g_variant_get (request_name_variant, "(u)", request_name_result);
+  g_variant_unref (request_name_variant);
 }
 
 static void