gitignore: add .DS_Store

gitignore: add .DS_Store

macOS creates .DS_Store files all over the place while browsing
directories. Add it to the list of ignored files.

Signed-off-by: Georgi Valkov <gvalkov@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/24942)

(cherry picked from commit 10c36d2f8d81a6f2b9a75f914fe094300835ba01)
(cherry picked from commit 97b2aa49e504e49a8862b89c65eb54e143395f1d)
(cherry picked from commit 1a869a65719f14f702dff492a6f90b135b9d0646)