perf tools: Fix session init on non-modular kernels

perf top and perf record refuses to initialize on non-modular kernels:
refuse to initialize:

 $ perf top -v
  map_groups__set_modules_path_dir: cannot open /lib/modules/2.6.33-rc6-tip-00586-g398dde3-dirty/

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1265223128-11786-1-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>