Fix nearly all warnings

src/emerge.h

@@ -31,8 +31,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 	infostream << "EmergeThread: " x << std::endl; }
 
 class Mapgen;
-class MapgenParams;
-class MapgenFactory;
+struct MapgenParams;
+struct MapgenFactory;
 class Biome;
 class BiomeDefManager;
 class EmergeThread;