Rename .di files to .d files to avoid linker errors.

When structs are used by value together with `.di` files this seems to create linker errors (e.g. using `FLAC__StreamMetadata metadata;`). I don't know whether this is a compiler bug or not, but the structs should be usable by value because they're fully-defined structs, not opaque structs.

Requesting merge: @andralex @WalterBright