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>