TextEditor: Use override consistently

clang-tidy fixes from modernize-use-override check.

Change-Id: Ie98c08279d7bf01f2705f631500dcabac5ba85d1
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/texteditor/syntaxhighlighter.h

@@ -58,7 +58,7 @@ public:
     SyntaxHighlighter(QObject *parent = 0);
     SyntaxHighlighter(QTextDocument *parent);
     SyntaxHighlighter(QTextEdit *parent);
-    virtual ~SyntaxHighlighter();
+    ~SyntaxHighlighter() override;
 
     void setDocument(QTextDocument *doc);
     QTextDocument *document() const;