unix: exclude AUDIT_RECORD_MAGIC constant on darwin

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>

unix/mkerrors.sh

@@ -428,6 +428,7 @@ ccflags="$@"
 		$2 ~ /^PERF_EVENT_IOC_/ ||
 		$2 ~ /^SECCOMP_MODE_/ ||
 		$2 ~ /^SPLICE_/ ||
+		$2 !~ /^AUDIT_RECORD_MAGIC/ &&
 		$2 ~ /^[A-Z0-9_]+_MAGIC2?$/ ||
 		$2 ~ /^(VM|VMADDR)_/ ||
 		$2 ~ /^IOCTL_VM_SOCKETS_/ ||