check for sys/pidfd.h

check for sys/pidfd.h

This header in newer glibc defines the signatures of functions
pidfd_send_signal() and pidfd_open() and when these functions are
defined by libc then we need to include the relevant header to get
the definitions. Clang 15+ has started to error out when function
signatures are missing.

Fixes errors like
misc-utils/kill.c:402:6: error: call to undeclared function 'pidfd_send_signal'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        if (pidfd_send_signal(pfd, ctl->numsig, &info, 0) < 0)

Signed-off-by: Khem Raj <raj.khem@gmail.com>