Valgrind: Add missing override modifiers.

Change-Id: Ief9f3f00678b73770dc7bbc431a9ddacf5f9b0f7
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/valgrind/memcheckengine.h

@@ -48,8 +48,8 @@ public:
     MemcheckRunControl(const Analyzer::AnalyzerStartParameters &sp,
         ProjectExplorer::RunConfiguration *runConfiguration);
 
-    bool startEngine();
-    void stopEngine();
+    bool startEngine() override;
+    void stopEngine() override;
 
     QStringList suppressionFiles() const;