examples/performance-thread: fix build with pkg-config

examples/performance-thread: fix build with pkg-config

main.c: In function ‘lthread_tx’:
main.c:2091:25: error: implicit declaration of function ‘sched_getcpu’;
 did you mean ‘sched_getparam’? [-Werror=implicit-function-declaration]
 2091 |  tx_conf->conf.cpu_id = sched_getcpu();
      |                         ^~~~~~~~~~~~
      |                         sched_getparam
cc1: all warnings being treated as errors

Explicitly pass _GNU_SOURCE and include missing header (rather than
rely on automagic inclusion from other system headers).

Fixes: d48415e1fee3 ("examples/performance-thread: add l3fwd-thread app")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>