edk2: Add .DS_Store to .gitignore for macOS

https://bugzilla.tianocore.org/show_bug.cgi?id=558

macOS may generate .DS_Store files in directories.
The .gitignore file is updated to ignore these
.DS_Store files.

Cc: Andrew Fish <afish@apple.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Andrew Fish <afish@apple.com>