package/procps-ng: fix build without __NR_pidfd_open

Fix the following build failure without __NR_pidfd_open raised since
bump to version 3.3.17 in commit
cc28c7aa and
https://gitlab.com/procps-ng/procps/-/commit/c8384e682c1cfb3b2dc797e0f8a3cbaaccf7a3da:

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

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



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>