Make SyntaxHighligher::_clear_highlighting_cache virtual

Since it's meant to be used as a virtual method.

scene/resources/syntax_highlighter.cpp

@@ -115,9 +115,8 @@ void SyntaxHighlighter::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("clear_highlighting_cache"), &SyntaxHighlighter::clear_highlighting_cache);
 	ClassDB::bind_method(D_METHOD("get_text_edit"), &SyntaxHighlighter::get_text_edit);
 
-	ClassDB::bind_method(D_METHOD("_clear_highlighting_cache"), &SyntaxHighlighter::_clear_highlighting_cache);
-
 	BIND_VMETHOD(MethodInfo(Variant::DICTIONARY, "_get_line_syntax_highlighting", PropertyInfo(Variant::INT, "p_line")));
+	BIND_VMETHOD(MethodInfo("_clear_highlighting_cache"));
 	BIND_VMETHOD(MethodInfo("_update_cache"));
 }