perf tools: Remove string.h from util.h

Not needed in this header, added to the places that need strdup,
strcmp and a few other prototypes.

Link: http://lkml.kernel.org/n/tip-t24yy85xnlv55kyosrum2ubs@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/namespaces.c

@@ -11,6 +11,7 @@
 #include "event.h"
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 
 struct namespaces *namespaces__new(struct namespaces_event *event)
 {