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>

1 files changed, 1 insertions, 0 deletions

diff --git a/utils/nfsynproxy.c b/utils/nfsynproxy.c
index baedc92c..bf5c4163 100644
--- a/utils/nfsynproxy.c
+++ b/utils/nfsynproxy.c
@@ -6,6 +6,7 @@
  * published by the Free Software Foundation.
  */
 
+#define _GNU_SOURCE
 #include <stdlib.h>
 #include <stdbool.h>
 #include <unistd.h>