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/comm.c

@@ -3,6 +3,7 @@
 #include <errno.h>
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <linux/refcount.h>
 
 struct comm_str {