perf tools: .gitignore += perf*.html

I've tried building the docs in tools/perf/Documentation/ , and after
that `git status` showed dozen of untracked htmls. Let's ignore them.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
LKML-Reference: <1253790022-10300-1-git-send-email-kirr@mns.spb.ru>
Signed-off-by: Ingo Molnar <mingo@elte.hu>