mingw: Don't redefine OPENVDB_EXPORT/OPENVDB_IMPORT
mingw: Don't redefine OPENVDB_EXPORT/OPENVDB_IMPORT

MinGW define both __GNUC__ and _WIN32.

Signed-off-by: Mehdi Chinoune <08b1b5066d15debe5d520804537f768887925fd1@hotmail.com>

dreamworksanimation/openvdb,dreamworksanimation/openvdb,dreamworksanimation/openvdb,dreamworksanimation/openvdb