IP_RECVTOS is not available on NetBSD

There is a patch for IP_RECVTOS under NetBSD because it does not exist there.

NetBSD Patch: https://cvsweb.netbsd.org/bsdweb.cgi/pkgsrc/net/fping/patches/patch-src_fping.c?rev=1.5;content-type=text%2Fplain