gcontenttype: Add no-op versions of {get,set}_mime_dirs() on win32/macOS
These are here to prevent linker errors, since gcontenttype.[ch]
aren’t compiled on Windows or macOS.
The implementations are stubs to be filled out by someone who knows each platform, at some point in the future.
Signed-off-by: Philip Withnall withnall@endlessm.com
Fixes: #1791 (closed)