perf tools: Fix _GNU_SOURCE macro related strndup() build error

strndup is a GNU extension. So dont include string.h without
defining _GNU_SOURCE (it results in a compile error otherwise).

Remove these includes as util.h does it already.
Signed-off-by: Liming Wang <liming.wang@windriver.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Cc: peterz@infradead.org
Cc: mhiramat@redhat.com
LKML-Reference: <1259734306-26323-1-git-send-email-liming.wang@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

tools/perf/util/string.c

-#include <string.h>
-#include <stdlib.h>
 #include "string.h"
 #include "util.h"