Clean up ObjectCache includes

components/resource/niffilemanager.cpp

@@ -1,5 +1,7 @@
 #include "niffilemanager.hpp"
 
+#include <osg/Object>
+
 #include <components/vfs/manager.hpp>
 
 #include "objectcache.hpp"

components/resource/objectcache.cpp

@@ -13,6 +13,8 @@
 
 #include "objectcache.hpp"
 
+#include <osg/Object>
+
 namespace Resource
 {
 

components/resource/objectcache.hpp

@@ -17,13 +17,18 @@
 #ifndef OPENMW_COMPONENTS_RESOURCE_OBJECTCACHE
 #define OPENMW_COMPONENTS_RESOURCE_OBJECTCACHE
 
-#include <osg/Node>
+#include <osg/Referenced>
-
+#include <osg/ref_ptr>
-#include <osgDB/ReaderWriter>
-#include <osgDB/DatabaseRevisions>
 
+#include <string>
 #include <map>
 
+namespace osg
+{
+    class Object;
+    class State;
+}
+
 namespace Resource {
 
 class ObjectCache : public osg::Referenced