Fix Metal includes breaking macOS local builds

Before this fix, skia (and thus Chrome) would fail to compile on macOS when the user (developer) had a case-sensitive file-system.

So I've replaced the incorrect includes of <metal/metal.h> by <Metal/Metal.h>

Change-Id: I6ebcc0f46608f6d840d80d18e5f5baf0744a7f16
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/225776
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/AUTHORS b/AUTHORS
index 192d69c..80776a6 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -11,6 +11,7 @@
 #
 # Please keep the list sorted alphabetically.
 
+Aaron O'Mullan <aaron.omullan@gmail.com>
 ACCESS CO., LTD. <*@access-company.com>
 Amazon, Inc <*@amazon.com>
 Anthony Catel <paraboul@gmail.com>