perf tools: Consider header files outside perf directory in tags target

This fixes lookups like "vi -t event_format"

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: http://lkml.kernel.org/r/20140528081918.GA28567@linutronix.de


Signed-off-by: Jiri Olsa <jolsa@kernel.org>