Add missing forward declarations.

git-svn-id: http://root.cern.ch/svn/root/trunk@21567 27541ba8-7e3a-0410-8455-c3a389f83636

geom/inc/TGeoBuilder.h

@@ -26,8 +26,12 @@
 
 class TGeoMaterial;
 class TGeoMatrix;
+class TGeoMedium;
 class TGeoShape;
 class TGeoVolume;
+class TGeoVolumeAssembly;
+class TGeoVolumeMulti;
+class TGeoManager;
 
 class TGeoBuilder : public TObject
 {