sys/capability.h is deprecated, use sys/capsicum.h instead.

Change-Id: I05a3b8ff28d4df6536a3f10e0da7381b48329738
Reviewed-on: https://go-review.googlesource.com/c/147737
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>