write(2) is defined in unistd.h.

For the iOS builder.

Change-Id: I411ffe81988d8fbafffde89e4732a20af1a63325
Reviewed-on: https://go-review.googlesource.com/37962
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>