perf_counter tools: Make .gitignore reflect perf_counter tools files

Make .gitignore reflect perf_counter tools files so
git status doesn't gripe about untracked files.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>