comparison of integers of different signs: 'gint' (aka 'int') and 'guint' (aka 'unsigned int')

Hi!

While building latest git master clang reported these errors:

  CC       libxfce4kbd_private_3_la-xfce-shortcuts.lo
xfce-shortcut-dialog.c:414:17: error: comparison of integers of different signs: 'gint' (aka 'int') and 'guint' (aka 'unsigned int') [-Werror,-Wsign-compare]
  for (i = 0; i < g_strv_length (keys); i++)
              ~ ^ ~~~~~~~~~~~~~~~~~~~~
1 error generated.
xfce-shortcuts-grabber.c:338:25: error: comparison of integers of different signs: 'gint' (aka 'int') and 'guint' (aka 'unsigned int') [-Werror,-Wsign-compare]
                  if (i >= key->keycodes->len)
                      ~ ^  ~~~~~~~~~~~~~~~~~~
make[3]: *** [Makefile:694: libxfce4kbd_private_3_la-xfce-shortcut-dialog.lo] Error 1