CL 112696 changed mkerrors.sh to add filesystem magic constants on
linux. This leads to the unrelated AUDIT_RECORD_MAGIC constant being
added on darwin. Avoid this by explicity excluding it in mkerrors.sh.

Change-Id: I5a73797e5e9a1a92cb39f96f360fdb48982a7741
Reviewed-on: https://go-review.googlesource.com/114095
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>