Apply patch tested by SuperMat to fix clang compilation

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@14649 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/states_screens/offline_kart_selection.cpp

@@ -11,3 +11,5 @@ OfflineKartSelectionScreen::~OfflineKartSelectionScreen()
 {
     //dtor
 }
+
+bool OfflineKartSelectionScreen::isRunning() { return singleton!=NULL; }

src/states_screens/offline_kart_selection.hpp

@@ -12,7 +12,7 @@ class OfflineKartSelectionScreen : public KartSelectionScreen, public GUIEngine:
         virtual ~OfflineKartSelectionScreen();
 
     public:
-        static bool isRunning() { return singleton!=NULL; }
+        static bool isRunning();
 
         // we do not override anything, this class is just there to have a singleton
 };