perf bench: Add new directory and header for new subcommand 'bench'

This patch adds bench/ directory and bench/bench.h.

bench/ directory will contain modules for bench subcommand.
bench/bench.h is for listing prototypes of module functions.

Signed-off-by: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: fweisbec@gmail.com
Cc: Jiri Kosina <jkosina@suse.cz>
LKML-Reference: <1257381097-4743-2-git-send-email-mitake@dcl.info.waseda.ac.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>