Add gstreamer-osx-m4-policy

[Tobias-Fischer]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

See e.g. #102, this is needed for conda-forge/gstreamer-feedstock#150

@wolfv @hmaarrfk - I've added both of you as users