Remove unused virtual modifiers in DebugHUD

/home/elsid/dev/openmw/components/sceneutil/mwshadowtechnique.cpp:3127:9: warning: Call to virtual function during construction [clang-analyzer-optin.cplusplus.VirtualCall]
        addAnotherShadowMap();
        ^
/home/elsid/dev/openmw/components/sceneutil/mwshadowtechnique.cpp:3094:5: note: Loop condition is true.  Entering loop body
    for (int i = 0; i < 2; ++i)
    ^
/home/elsid/dev/openmw/components/sceneutil/mwshadowtechnique.cpp:3094:5: note: Loop condition is true.  Entering loop body
/home/elsid/dev/openmw/components/sceneutil/mwshadowtechnique.cpp:3094:5: note: Loop condition is false. Execution continues on line 3102
/home/elsid/dev/openmw/components/sceneutil/mwshadowtechnique.cpp:3126:21: note: Assuming 'i' is < 'numberOfShadowMapsPerLight'
    for (int i = 0; i < numberOfShadowMapsPerLight; ++i)
                    ^
/home/elsid/dev/openmw/components/sceneutil/mwshadowtechnique.cpp:3126:5: note: Loop condition is true.  Entering loop body
    for (int i = 0; i < numberOfShadowMapsPerLight; ++i)
    ^
/home/elsid/dev/openmw/components/sceneutil/mwshadowtechnique.cpp:3127:9: note: This constructor of an object of type 'DebugHUD' has not returned when the virtual method was called
        addAnotherShadowMap();
        ^
/home/elsid/dev/openmw/components/sceneutil/mwshadowtechnique.cpp:3127:9: note: Call to virtual function during construction

components/sceneutil/mwshadowtechnique.hpp

@@ -261,18 +261,18 @@ namespace SceneUtil {
 
         float                                   _shadowFadeStart = 0.0;
 
-        class DebugHUD : public osg::Referenced
+        class DebugHUD final : public osg::Referenced
         {
         public:
             DebugHUD(int numberOfShadowMapsPerLight);
 
-            virtual void draw(osg::ref_ptr<osg::Texture2D> texture, unsigned int shadowMapNumber, const osg::Matrixd &matrix, osgUtil::CullVisitor& cv);
+            void draw(osg::ref_ptr<osg::Texture2D> texture, unsigned int shadowMapNumber, const osg::Matrixd &matrix, osgUtil::CullVisitor& cv);
 
-            virtual void releaseGLObjects(osg::State* state = 0) const;
+            void releaseGLObjects(osg::State* state = 0) const;
 
-            virtual void setFrustumVertices(osg::ref_ptr<osg::Vec3Array> vertices, unsigned int traversalNumber);
+            void setFrustumVertices(osg::ref_ptr<osg::Vec3Array> vertices, unsigned int traversalNumber);
         protected:
-            virtual void addAnotherShadowMap();
+            void addAnotherShadowMap();
 
             static const int sDebugTextureUnit = 0;