perf_event_open.2: 4.1 clockid support

perf_event_open.2: 4.1 clockid support

This manpage patch relates to the addition of the clockid
functionaliry added in the following commit:

	commit 34f439278cef7b1177f8ce24f9fc81dfc6221d3b
	Author: Peter Zijlstra <peterz@infradead.org>

	perf: Add per event clockid support

	Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
	Cc: Andrew Morton <akpm@linux-foundation.org>
	Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
	Cc: David Ahern <dsahern@gmail.com>
	Cc: Jiri Olsa <jolsa@redhat.com>
	Cc: John Stultz <john.stultz@linaro.org>
	Cc: Linus Torvalds <torvalds@linux-foundation.org>
	Cc: Peter Zijlstra <peterz@infradead.org>
	Cc: Stephane Eranian <eranian@google.com>
	Cc: Thomas Gleixner <tglx@linutronix.de>
	Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>