perf tools: Remove string.h, unistd.h and sys/stat.h from util.h

Not needed in this header, added to the places that need FILE,
putchar(), access() and a few other prototypes.

Link: http://lkml.kernel.org/n/tip-xxtdsl6nsna82j7puwbdjqhs@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/bpf.c

@@ -1,6 +1,8 @@
 #include <errno.h>
 #include <stdio.h>
 #include <sys/epoll.h>
+#include <sys/types.h>
+#include <sys/stat.h>
 #include <util/util.h>
 #include <util/bpf-loader.h>
 #include <util/evlist.h>