marfa.h: fix MSVC warning about 'assignment operator could not be generated
marfa.h: fix MSVC warning about 'assignment operator could not be generated

Found during GDAL testing:

https://github.com/OSGeo/gdal/commit/af1aa6789ed61fbfd28bda9c3123278845a11808

nasa-gibs/mrf,nasa-gibs/mrf,nasa-gibs/mrf