xtables-monitor: fix build with older glibc

xtables-monitor: fix build with older glibc

glibc older than 2.19 only expose BSD style fields of struct tcphdr when
_BSD_SOURCE is define. Current glibc however, warn that _BSD_SOURCE is
deprecated. Migrate to the GNU style of tcphdr fields to make the code
compatible with any glibc version.

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 'th_sport'
    printf("SPORT=%d DPORT=%d ", ntohs(tcph->th_sport), ntohs(tcph->th_dport));
                                           ^
xtables-monitor.c:406:66: error: 'const struct tcphdr' has no member named 'th_dport'
    printf("SPORT=%d DPORT=%d ", ntohs(tcph->th_sport), ntohs(tcph->th_dport));
                                                                  ^
...

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