fix pifd_open check

fix pifd_open check

Replace AC_CHECK_FUNC by AC_CHECK_FUNCS otherwise HAVE_PIDFD_OPEN will
never be defined resulting in the following build failure if pidfd_open
is available but __NR_pidfd_open is not available:

pgrep.c: In function 'pidfd_open':
pgrep.c:748:17: error: '__NR_pidfd_open' undeclared (first use in this function); did you mean 'pidfd_open'?
  748 |  return syscall(__NR_pidfd_open, pid, flags);
      |                 ^~~~~~~~~~~~~~~
      |                 pidfd_open

This build failure is raised since the addition of pwait in version
3.3.17 and
https://gitlab.com/procps-ng/procps/-/commit/c8384e682c1cfb3b2dc797e0f8a3cbaaccf7a3da

Fixes:
 - http://autobuild.buildroot.org/results/f23a5156e641b2ebdd673973dec0f9c87760c688

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>