Fix virtual functions in Linux PolycodePlayer

I got this error while executing BuildLinux.sh:

Scanning dependencies of target PolycodePlayer
[ 95%] Building CXX object Player/Contents/CMakeFiles/PolycodePlayer.dir/Source/PolycodePlayer.cpp.o
[ 95%] Building CXX object Player/Contents/CMakeFiles/PolycodePlayer.dir/Source/PolycodeLinuxPlayer.cpp.o                                                                          
/home/xxxxxxxxxx/workspace/Polycode/Player/Contents/Source/PolycodeLinuxPlayer.cpp: In member function ‘virtual void PolycodeLinuxPlayer::createCore()’:
/home/xxxxxxxxxx/workspace/Polycode/Player/Contents/Source/PolycodeLinuxPlayer.cpp:34:81: error: invalid new-expression of abstract class type ‘Polycode::SDLCore’
  core =  new SDLCore(view, xRes, yRes,  fullScreen, false, aaLevel, 0, frameRate); 
                                                                                 ^
In file included from /home/xxxxxxxxxx/workspace/Polycode/Core/Contents/PolycodeView/Linux/PolycodeView.h:3:0,
                 from /home/xxxxxxxxxx/workspace/Polycode/Player/Contents/Include/PolycodeLinuxPlayer.h:25,
                 from /home/xxxxxxxxxx/workspace/Polycode/Player/Contents/Source/PolycodeLinuxPlayer.cpp:22:
/home/xxxxxxxxxx/workspace/Polycode/Core/Contents/Include/PolySDLCore.h:42:20: note:   because the following virtual functions are pure within ‘Polycode::SDLCore’:
  class _PolyExport SDLCore : public Core {
                    ^
In file included from /home/xxxxxxxxxx/workspace/Polycode/Core/Contents/Include/PolySDLCore.h:26:0,
                 from /home/xxxxxxxxxx/workspace/Polycode/Core/Contents/PolycodeView/Linux/PolycodeView.h:3,
                 from /home/xxxxxxxxxx/workspace/Polycode/Player/Contents/Include/PolycodeLinuxPlayer.h:25,
                 from /home/xxxxxxxxxx/workspace/Polycode/Player/Contents/Source/PolycodeLinuxPlayer.cpp:22:
/home/xxxxxxxxxx/workspace/Polycode/Core/Contents/Include/PolyCore.h:99:22: note:      virtual bool Polycode::Core::systemUpdate()
         virtual bool systemUpdate() = 0;
                      ^
/home/xxxxxxxxxx/workspace/Polycode/Core/Contents/Include/PolyCore.h:282:18: note:     virtual Polycode::String Polycode::Core::saveFilePicker(std::vector<Polycode::CoreFileExtension>)
   virtual String saveFilePicker(std::vector<CoreFileExtension> extensions) = 0;
                  ^
/home/xxxxxxxxxx/workspace/Polycode/Core/Contents/Include/PolyCore.h:292:16: note:     virtual void Polycode::Core::setVideoMode(int, int, bool, bool, int, int, bool)
   virtual void setVideoMode(int xRes, int yRes, bool fullScreen, bool vSync, int aaLevel, int anisotropyLevel, bool retinaSupport=true) = 0;
                ^
Player/Contents/CMakeFiles/PolycodePlayer.dir/build.make:80: recipe for target 'Player/Contents/CMakeFiles/PolycodePlayer.dir/Source/PolycodeLinuxPlayer.cpp.o' failed
make[2]: *** [Player/Contents/CMakeFiles/PolycodePlayer.dir/Source/PolycodeLinuxPlayer.cpp.o] Error 1
CMakeFiles/Makefile2:774: recipe for target 'Player/Contents/CMakeFiles/PolycodePlayer.dir/all' failed
make[1]: *** [Player/Contents/CMakeFiles/PolycodePlayer.dir/all] Error 2
Makefile:136: recipe for target 'all' failed
make: *** [all] Error 2