org-element: Autoload org-element-use-cache

* lisp/org-element.el (org-element-use-cache): Autoload to silence
byte compiler warning in Emacs repo.

On Emacs's emacs-29 branch, compiling lisp/org/org-loaddefs.el gives
the following warning that stems from an autoloaded defsubst,
org-element-at-point-no-context (new to Org 9.6):

  In org-element-at-point-no-context:
  org/org-loaddefs.el:846:61: Warning: Unused lexical variable
  `org-element-use-cache'

That doesn't show up in the Org repo because mk/org-fixup.el generates
org-loaddefs.el via autoload.el's generate-file-autoloads, which adds
"no-byte-compile: t" to its output file.  However, in Emacs 29,
autoload.el is now deprecated, and the Emacs build uses
loaddefs-gen.el's loaddefs-generate--emacs-batch, which does include
no-byte-compile.

lisp/org-element.el

@@ -5295,6 +5295,7 @@ indentation removed from its contents."
 ;; `org-element--cache-diagnostics-ring-size', `org-element--cache-map-statistics',
 ;; `org-element--cache-map-statistics-threshold'.
 
+;;;###autoload
 (defvar org-element-use-cache t
   "Non-nil when Org parser should cache its results.")