perf tools: Ignore auto-generated bison/flex files

perf tools: Ignore auto-generated bison/flex files

The commit 5fba73f5d1e0 ("perf tools: Remove auto-generated bison/flex
files") removed those files from git, so they'll be listed on untracked
files after building perf. Fix it.

Signed-off-by: Namhyung Kim <namhyung.kim@lge.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1333948274-20043-1-git-send-email-namhyung.kim@lge.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>