perf tools: Remove duplicated string.h includes

tools/perf/util/probe-event.c included 'string.h' twice, remove the
duplicate.

Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Danny Kukawka <dkukawka@suse.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jovi Zhang <bookjovi@gmail.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1329400459-31570-1-git-send-email-danny.kukawka@bisect.de


Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>