Debugger: Don't show a location marker in the mixed master engine

Task-number: QTCREATORBUG-14087
Change-Id: I7e8f116c97cdea0bd8234d060d068435229e6fc2
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/debugger/gdb/gdbengine.h

@@ -72,6 +72,7 @@ public:
     ~GdbEngine();
 
 private: ////////// General Interface //////////
+    virtual DebuggerEngine *cppEngine() { return this; }
 
     virtual void setupEngine() = 0;
     virtual void handleGdbStartFailed();