Fix signedness warning in gio/gio-tool.c:attribute_flags_to_string()

gio/gio-tool.c: In function ‘attribute_flags_to_string’:
gio/gio-tool.c:171:17: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’
  171 |   for (i = 0; i < G_N_ELEMENTS (flag_descr); i++)
      |                 ^