perf_counter tools: Give perf top inherit option

Currently, perf top -p only tracks the pid provided, which isn't very useful
for watching forky loads, so give it an inherit option.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1248165036.9795.10.camel@marge.simson.net>