Add .DS_Store to MuJoCo's .gitignore.
Add .DS_Store to MuJoCo's .gitignore.

.DS_Store is created by macOS.

PiperOrigin-RevId: 436477360
Change-Id: I9b3e72d89916bcaea88f3f34f73bf211dabb8467

deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco