multimedia/arcan: fix a warning on FreeBSD < 12

src/frameserver/util/anet_keystore_naive.c:231:10: warning: implicit declaration of function 'getline' is invalid in C99 [-Wimplicit-function-declaration]

while (getline(&inbuf, &len, fpek) != -1){
       ^

src/frameserver/util/anet_keystore_naive.c:427:15: warning: implicit declaration of function 'getline' is invalid in C99 [-Wimplicit-function-declaration]

while ((nr = getline(&inbuf, &len, fpek)) != -1){
             ^