BUILD: threads: fix -Wundef for _POSIX_PRIORITY_SCHEDULING on libmusl

Building with an old musl-based toolchain reported this warning:

  include/haproxy/thread.h: In function 'ha_thread_relax':
  include/haproxy/thread.h:256:5: warning: "_POSIX_PRIORITY_SCHEDULING" is not defined [-Wundef]
   #if _POSIX_PRIORITY_SCHEDULING
       ^

There were indeed two "#if" insteadd of #ifdef" for this macro, let's
fix them.

(cherry picked from commit 8ab941939480af65660182e343df22fcdced6005)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed