gvc: squash 'api_t' related -Wsign-compare warnings

gvc: squash 'api_t' related -Wsign-compare warnings

The range of either 'int' or 'size_t' is enough to cover all valid values this
takes on, so we can safely squash the following:

  gvplugin.c: In function ‘gvPluginList’:
  gvplugin.c:408:23: warning: comparison of integer expressions of different
    signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
    408 |     for (api = 0; api < ARRAY_SIZE(api_names); api++) {
        |                       ^
  gvplugin.c: In function ‘gvplugin_write_status’:
  gvplugin.c:450:23: warning: comparison of integer expressions of different
    signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
    450 |     for (api = 0; api < ARRAY_SIZE(api_names); api++) {
        |                       ^
  gvplugin.c: In function ‘gvplugin_graph’:
  gvplugin.c:501:27: warning: comparison of integer expressions of different
    signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
    501 |         for (api = 0; api < ARRAY_SIZE(api_names); api++) {
        |                           ^
  gvplugin.c:680:27: warning: comparison of integer expressions of different
    signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
    680 |         for (api = 0; api < ARRAY_SIZE(api_names); api++) {
        |                           ^