ebib-preload-bib-files: autoload safety predicate

This is useful because the variable definition, including the `:safe:'
declaration, is only loaded as the package is loaded. If the user sets
the variable locally (e.g. in `Local Variables') and then visits that
file in a new session before opening ebib for the first time, the safe
predicate won't be recognised and the user is prompted for whether to
use the new value. Using an autoload is the solution to this suggested
in the elisp manual `(elisp) File Local Variables'.

ebib-utils.el

@@ -69,13 +69,13 @@ This is the entry type assigned to newly created entries."
   :group 'ebib
   :type 'string)
 
+;;;###autoload (put 'ebib-preload-bib-files 'safe-local-variable '(lambda (val) (seq-every-p #'stringp val)))
 (defcustom ebib-preload-bib-files nil
   "List of BibTeX files to load automatically when Ebib starts.
 This option allows you to specify which `.bib' file(s) Ebib
 should load automatically when it starts up.  Specify one file per
 line.  You can complete a partial filename with `M-TAB`."
   :group 'ebib
-  :safe (lambda (val) (seq-every-p #'stringp val))
   :type '(repeat (file :must-match t)))
 
 (defcustom ebib-local-bibfiles t