elisp-common.eclass: fix for Emacs 29 (explicitly require autoload)

Emacs 29's NEWS says: "The autoload.el library is now obsolete."

We should therefore explicitly 'require it in order to have the
eclass machinery work.

Fix build failures with e.g. exec-path-from-shell, flycheck:
```
 * Generating autoload file for GNU Emacs ...

Error: void-function (batch-update-autoloads)
  debug-early-backtrace()
  debug-early(error (void-function batch-update-autoloads))
  batch-update-autoloads()
  command-line-1(("--eval" "(setq make-backup-files nil)" "--eval" "(setq generated-autoload-file (expand-file-name \"exec-path-from-shell-autoloads.el\"))" "-f" "batch-update-autoloads" "."))
  command-line()
  normal-top-level()
Symbol’s function definition is void: batch-update-autoloads
 * elisp-make-autoload-file: batch-update-autoloads failed
 [ !! ]
 * ERROR: app-emacs/exec-path-from-shell-1.12::gentoo failed (compile phase):
 *   (no error message)
```

See https://www.mail-archive.com/freebsd-pkg-fallout@freebsd.org/msg1982134.html
and https://lists.gnu.org/archive/html/bug-gnu-emacs/2022-08/msg01259.html.

Thanks-to: John Turner <jturner.usa+gentoo@gmail.com>
Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Ulrich Müller <ulm@gentoo.org>

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