org-open-at-mouse: Suppress warning in Emacs <30

* lisp/org.el (org-open-at-mouse): Add variable declaration to silence
warnings in older Emacs.

lisp/org.el

@@ -8278,9 +8278,12 @@ See the docstring of `org-open-file' for details."
   ;; link abbreviations. So, suppressing parser complains about
   ;; non-Org buffer to keep the feature working at least to the extent
   ;; it did before.
+  (defvar warning-suppress-types) ; warnings.el
   (let ((warning-suppress-types
          (cons '(org-element org-element-parser)
                warning-suppress-types)))
+    ;; FIXME: Suppress warning in Emacs <30
+    ;; (ignore warning-suppress-types)
     (org-open-at-point)))
 
 (defvar org-window-config-before-follow-link nil