QSGTexture::nativeInterface() doesn't work for QSGMetalTexture

QSGTexture::nativeInterface<QNativeInterface::QSGMetalTexture>() always returns null because the relevant branch is never compiled due to the OBJC flag which is never defined in a CPP file.

https://github.com/qt/qtdeclarative/blob/dev/src/quick/scenegraph/coreapi/qsgtexture.cpp#L977-L979

#if defined(_OBJC_)
QT_NATIVE_INTERFACE_RETURN_IF(QSGMetalTexture, &dd->m_metalTextureAccessor);
#endif