.gitignore: ignore .DS_Store files.  [skip ci]

macOS's Finder drops them in directories it looks at (if it has write
permission); it's just data to remember how the Finder displayed the
directory, it's not of any interest to Git or to Git users.

(cherry picked from commit 9ee3db44cf597ce5dbc3ac34d351935ae1f61252)
diff --git a/.gitignore b/.gitignore
index d4b8456..df7b440 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
+.DS_Store
 Makefile
 *~
 *.o