Declare function, silence byte compiler

* org-footnote.el (org-id-uuid): Declare function, silence byte
  compiler.

lisp/org-footnote.el

@@ -48,6 +48,7 @@
 (declare-function org-end-of-subtree "org"  (&optional invisible-ok to-heading))
 (declare-function org-in-verbatim-emphasis "org" ())
 (declare-function org-inside-latex-macro-p "org" ())
+(declare-function org-id-uuid "org" ())
 (defvar org-odd-levels-only) ;; defined in org.el
 (defvar message-signature-separator) ;; defined in message.el