util: fix compilation on macos

timespec_get() is not available on macos, we need to pull in the
include/c11/threads_posix.h helper.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103674


Fixes: e2d761de ("util: drop final reference to p_compiler.h")
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 9d3fc737)