Add .DS_Store to skcms ignore list.

.DS_Store is automatically created by Mac OS when browsing the file
system.

Change-Id: I6b477ce92b99c7a2acbda1916a42dea680155ed7
Bug: skia:12849
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/497441
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
diff --git a/.gitignore b/.gitignore
index d0c0792..4c45e72 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
+.DS_Store
 out
 *.swp
 bazel-*