org-footnote: forward declare functions

* lisp/org-footnote.el: Forward declare `org-element-context',
  `org-element-property' and `org-element-type'.

Introduced with 51e1131ff8.

lisp/org-footnote.el

@@ -41,6 +41,9 @@
 (declare-function org-back-over-empty-lines "org" ())
 (declare-function org-back-to-heading "org" (&optional invisible-ok))
 (declare-function org-combine-plists "org" (&rest plists))
+(declare-function org-element-context "org-element" (&optional element))
+(declare-function org-element-property "org-element" (property element))
+(declare-function org-element-type "org-element" (element))
 (declare-function org-end-of-subtree "org"  (&optional invisible-ok to-heading))
 (declare-function org-fill-paragraph "org" (&optional justify))
 (declare-function org-icompleting-read "org" (&rest args))