Fix signedness warning in gio/gdummyfile.c

gio/gdummyfile.c: In function ‘unescape_string’:
gio/gdummyfile.c:485:32: error: comparison of integer expressions of different signedness: ‘long int’ and ‘size_t’ {aka ‘long unsigned int’}
  485 |   g_warn_if_fail (out - result <= strlen (escaped_string));
      |                                ^~

gio/gdummyfile.c

@@ -482,7 +482,7 @@ unescape_string (const gchar *escaped_string,
     }
   
   *out = '\0';
-  g_warn_if_fail (out - result <= strlen (escaped_string));
+  g_warn_if_fail ((gsize) (out - result) <= strlen (escaped_string));
   return result;
 }