perf tools: No need to include internal/lib.h from util/util.h

That was done just to have users of writen() and readn(), that before
had their prototypes in util/util.h to get it without having to add an
include for internal/lib.h, but the right way is to add it and by now
all places already do it.

Fix a fallout were readlink() was used but unistd.h was being obtained
by luck thru util.h -> internal/lib.h, now to check why unistd.h is
being included there...

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-lcnytgrtafey3kwlfog2rzzj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/sdt.c

@@ -3,6 +3,7 @@
 #include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 #include <sys/epoll.h>
 #include <util/symbol.h>
 #include <linux/filter.h>