perf tools: Add missing tools/perf/util/include/string.h

To cure a bunch of:

In file included from util/include/linux/bitmap.h:1,
from util/header.h:8,
from builtin-trace.c:7:
util/include/../../../../include/linux/bitmap.h:8:26: error:
linux/string.h: No such file or directory make: ***
[builtin-trace.o] Error 1 make: *** Waiting for unfinished
jobs....

Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>
Acked-by: Frederic Weisbecker <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
LKML-Reference: <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

tools/perf/Makefile

···
 LIB_H += ../../include/linux/rbtree.h
 LIB_H += ../../include/linux/list.h
 LIB_H += util/include/linux/list.h
+LIB_H += util/include/linux/string.h
 LIB_H += perf.h
 LIB_H += util/event.h
 LIB_H += util/types.h

tools/perf/util/include/linux/string.h

···
+#include <string.h>