net/widentd: fix build on ARM / POWER

widentd.c:70:48: error: result of comparison of constant -1 with expression of type 'char' is always true [-Werror,-Wtautological-constant-out-of-range-compare]

while ((ch = getopt(argc, argv, "vo:u:s:i:")) != -1)
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~