Also return display DPI from wxScreenDC in wxGTK3

This is similar to the previous commit and is done for the same reasons:
screen DC needs to use the same DPI as the screen itself, instead of the
default Cairo 72 DPI.

include/wx/gtk/dc.h

@@ -76,6 +76,8 @@ class wxScreenDCImpl: public wxGTKCairoDCImpl
 public:
     wxScreenDCImpl(wxScreenDC* owner);
 
+    virtual wxSize GetPPI() const wxOVERRIDE;
+
     wxDECLARE_NO_COPY_CLASS(wxScreenDCImpl);
 };
 //-----------------------------------------------------------------------------