org-export-ignored-local-variables: Do not copy cache

* lisp/ox.el (org-export-ignored-local-variables): Do not copy buffer
cache.  It is large by size (and, thus, slow to `read') and may be
changed destructively by reference, interfering with the original
buffer when calling `org-export-copy-buffer'.

Reported-by: Rudolf Adamkovič <salutis@me.com>
Link: https://list.orgmode.org/m2zgef774u.fsf@me.com/T/#t

lisp/ox.el

@@ -264,11 +264,13 @@ See `org-export-inline-image-p' for more information about
 rules.")
 
 (defconst org-export-ignored-local-variables
-  '(org-font-lock-keywords
-    org-element--cache-change-tic org-element--cache-change-tic org-element--cache-size
-    org-element--cache-sync-keys-value org-element--cache-change-warning
-    org-element--cache org-element--cache-objects org-element--cache-sync-keys
-    org-element--cache-sync-requests org-element--cache-sync-timer)
+  '( org-font-lock-keywords org-element--cache-change-tic
+     org-element--cache-change-tic org-element--cache-size
+     org-element--headline-cache-size
+     org-element--cache-sync-keys-value
+     org-element--cache-change-warning org-element--headline-cache
+     org-element--cache org-element--cache-sync-keys
+     org-element--cache-sync-requests org-element--cache-sync-timer)
   "List of variables not copied through upon buffer duplication.
 Export process takes place on a copy of the original buffer.
 When this copy is created, all Org related local variables not in