Fix -Wnon-virtual-dtor warnings.

Example of the warning:
./core/script_language.h:198:7: warning: 'class ScriptCodeCompletionCache' has virtual functions and accessible non-virtual destructor [-Wnon-virtual-dtor]

(cherry picked from commit f9f2413e69)

scene/gui/text_edit.h

@@ -683,6 +683,7 @@ protected:
 	TextEdit *text_editor;
 
 public:
+	virtual ~SyntaxHighlighter() {}
 	virtual void _update_cache() = 0;
 	virtual Map<int, TextEdit::HighlighterInfo> _get_line_syntax_highlighting(int p_line) = 0;