Fix platform compiles so that dracodec_unity.bundle only applies to OSX.

Very simple fix to correctly set the meta for dracodec_unity.bundle to only apply to OSX.

Prior to this fix, it's set to include all platforms. Normally this doesn't matter as the files get ignored, but when pushing to Apple (TestFlight) it causes an invalid binary error.

Including the full message from Apple below just to help google-ability for anyone running into a similar issue in Unity.

ITMS-90535: Unexpected CFBundleExecutable Key - The bundle at 'Payload/my-app.app/Frameworks/UnityFramework.framework/dracodec_unity.bundle' does not contain a bundle executable. If this bundle intentionally does not contain an executable, consider removing the CFBundleExecutable key from its Info.plist and using a CFBundlePackageType of BNDL. If this bundle is part of a third-party framework, consider contacting the developer of the framework for an update to address this issue.