utils: nfsynproxy: fix build with musl libc

utils: nfsynproxy: fix build with musl libc

The musl libc exposes some struct tcphdr field only when _GNU_SOURCE is
defined. Fix the following build failure:

nfsynproxy.c: In function ‘parse_packet’:
nfsynproxy.c:34:9: error: ‘const struct tcphdr’ has no member named ‘syn’
  if (!th->syn || !th->ack)
         ^
nfsynproxy.c:34:21: error: ‘const struct tcphdr’ has no member named ‘ack’
  if (!th->syn || !th->ack)
                     ^
nfsynproxy.c:42:8: error: ‘const struct tcphdr’ has no member named ‘res2’
  if (th->res2 == 0x1)
        ^
nfsynproxy.c:45:13: error: ‘const struct tcphdr’ has no member named ‘doff’
  length = th->doff * 4 - sizeof(*th);
             ^

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>