Windows build: add hack so that OGRMutexedLayer and OGRMutexedDataSource classes are exported in the import library, to fix FileGDB plugin build. The reason for what this is now necessary (apparently since the merge of curve geometry support) is completely a mystery

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27972 f0d54148-0727-0410-94bb-9a71ac55c965