CLEANUP: haproxy: using _GNU_SOURCE instead of __USE_GNU macro.

CLEANUP: haproxy: using _GNU_SOURCE instead of __USE_GNU macro.

In order to properly enable sched_setaffinity, in some versions of Linux,
it is rather _GNU_SOURCE than __USE_GNU (spotted on Alpine Linux for instance),
also for the sake of consistency as __USE_GNU seems not used across the code and
for last, it seems on Linux it is the best way to enable non portable code.
On Linux glibc's based versions, it seems _GNU_SOURCE defines __USE_GNU
it should be safe enough.