* 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.
