improve .gitignore for MacOS

Some MacOS apps (the finder in particular) cause .DS_Store files to be
created. Avoid cluttering up git status with these files.

Change-Id: I76ec7aa9657ade812513567c1b0c58b67d126dcc
diff --git a/.gitignore b/.gitignore
index 53191fd..ebef272 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 /.v23
+**/.DS_Store