Add CLONE_PIDFD from Linux 5.2 to bits/sched.h.

This patch adds the CLONE_PIDFD constant from Linux 5.2 to glibc's
bits/sched.h.

Tested for x86_64.

	* sysdeps/unix/sysv/linux/bits/sched.h [__USE_GNU] (CLONE_PIDFD):
	New macro.