coreaudio-encoder: Fix cmake for mingw

This prevents windres to catch up '-Wno-multichar' as an option which is
unknown to it.  This flag was added in commit
aa0e64b7c9
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f