procps: fix build with new glibc but old kernel headers

If you're building procps with a newer glibc (with pidfd_open()) but
older kernel headers (say 4.x, before __NR_pidfd_open) then procps will
fail to build because of a typo in configure.ac.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>