Fix shader crash when the comma used in for loop as a trailing

(cherry picked from commit d74749fd60)

servers/rendering/shader_language.h

@@ -1037,6 +1037,7 @@ private:
 	Token _make_token(TokenType p_type, const StringName &p_text = StringName());
 	Token _get_token();
 	bool _lookup_next(Token &r_tk);
+	Token _peek();
 
 	ShaderNode *shader = nullptr;