Debugger: Connect GdbServerPortsGatherer signals in constructor

Not in start(), when it would add extra connections on each re-run.

Change-Id: I5fe9e047a188276f3bde54227dc0509ab77366c4
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/debugger/debuggerruncontrol.h

@@ -111,7 +111,8 @@ public:
     Utils::Port qmlServerPort() const { return m_qmlServerPort; }
 
 private:
-    void start();
+    void start() override;
+    void handlePortListReady();
 
     ProjectExplorer::DeviceUsedPortsGatherer m_portsGatherer;
     bool m_useGdbServer = false;