fix metal build errors with preprocessor definitions

Bug: skia:8243

Change-Id: Ic312a1d87cc2eba8f0fe7f66c16e45c4bfe2ef49
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201540
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
diff --git a/AUTHORS b/AUTHORS
index a66487d..0e405b7 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -56,3 +56,4 @@
 Yong-Hwan Baek <meisterdevhwan@gmail.com>
 Alexander Khovansky <alx.khovansky@gmail.com>
 Zhuo Qingliang <zhuo.dev@gmail.com>
+Mainframe North <*@mainframe.co.uk>