perf tools: Include errno.h where needed

Removing it from util.h, part of an effort to disentangle the includes
hell, that makes changes to util.h or something included by it to cause
a complete rebuild of the tools.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-ztrjy52q1rqcchuy3rubfgt2@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/tests/openat-syscall.c b/tools/perf/tests/openat-syscall.c
index 5964938..5f0c700 100644
--- a/tools/perf/tests/openat-syscall.c
+++ b/tools/perf/tests/openat-syscall.c
@@ -1,3 +1,4 @@
+#include <errno.h>
 #include <inttypes.h>
 #include <api/fs/tracing_path.h>
 #include <linux/err.h>