xtables-monitor: fix build with musl libc

Commit 7c8791edac3 ("xtables-monitor: fix build with older glibc")
changed the code to use GNU style tcphdr fields. Unfortunately, musl
libc requires _GNU_SOURCE definition to expose these fields.

Fix the following build failure:

xtables-monitor.c: In function ‘trace_print_packet’:
xtables-monitor.c:406:43: error: ‘const struct tcphdr’ has no member named ‘source’
    printf("SPORT=%d DPORT=%d ", ntohs(tcph->source), ntohs(tcph->dest));
                                           ^~
xtables-monitor.c:406:64: error: ‘const struct tcphdr’ has no member named ‘dest’
    printf("SPORT=%d DPORT=%d ", ntohs(tcph->source), ntohs(tcph->dest));
                                                                ^~
...

Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Florian Westphal <fw@strlen.de>

0 files changed, 0 insertions, 0 deletions