Defining _GNU_SOURCE is required to get proper member names in `struct udphdr`
when building against musl. Build error:

  util.c: In function 'RawFillHeaders':
  util.c:481:9: error: 'struct udphdr' has no member named 'source'
       udph->source = htons(portsrc);
           ^
  util.c:482:9: error: 'struct udphdr' has no member named 'dest'
       udph->dest   = htons(portdst);
           ^
  util.c:483:9: error: 'struct udphdr' has no member named 'len'
       udph->len    = htons(sizeof(struct udphdr) + len);
           ^
  util.c:484:9: error: 'struct udphdr' has no member named 'check'
       udph->check  = 0;

Including <sys/types.h> in needed for mode_t, otherwise musl build fails:
  util.h:91:1: error: unknown type name 'mode_t'

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>