perf/x86: Make merge_attr() global to use from perf_event_intel

perf/x86: Make merge_attr() global to use from perf_event_intel

merge_attr() allows to merge two sysfs attribute tables.
Export it to be usable by other files too.

Next patch is going to use that to extend the sysfs format
attributes for a CPU.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: eranian@google.com
Link: http://lkml.kernel.org/r/1435612935-24425-1-git-send-email-andi@firstfloor.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>