Add TFD_TIMER_CANCEL_ON_SET to sys/timerfd.h.

In Linux 4.10, timerfd constants moved to a new uapi header, which
showed up that glibc's sys/timerfd.h is missing the old flag
TFD_TIMER_CANCEL_ON_SET.  This patch adds that flag to glibc's header.

Tested for x86_64.

	* sysdeps/unix/sysv/linux/sys/timerfd.h (TFD_TIMER_CANCEL_ON_SET):
	New enum constant and macro.