perf_counter tools: Optionally pass a symbol filter to the dso load routines

Will be used by perf top.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090528175526.GF4747@ghostprotocols.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>