GUI: Small cleanup - use const reference.

gui/checkthread.cpp

@@ -35,7 +35,7 @@ CheckThread::~CheckThread()
     //dtor
 }
 
-void CheckThread::Check(Settings settings)
+void CheckThread::Check(const Settings &settings)
 {
     mCppcheck.settings(settings);
     start();

gui/checkthread.h

@@ -44,7 +44,7 @@ public:
     *
     * @param settings settings for cppcheck
     */
-    void Check(Settings settings);
+    void Check(const Settings &settings);
 
     /**
     * @brief method that is run in a thread