unix: remove Flock syscall for aix

AIX doesn't provide a flock() syscall, it was previously emulated in
package syscall by using fcntl. This emulation was removed in CL 152397.
Since unix.Flock wraps syscall.Flock, the build currently fails on aix
without this change.

Change-Id: Ie887619cd64ae009ea43f00c74e450a5b8f998ac
Reviewed-on: https://go-review.googlesource.com/c/153938
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Clément Chigot <clement.chigot@atos.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/example_exec_test.go

@@ -0,0 +1,19 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package unix_test
+
+import (
+	"log"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+func ExampleExec() {
+	err := unix.Exec("/bin/ls", []string{"ls", "-al"}, os.Environ())
+	log.Fatal(err)
+}