org-element: Do not reset `org-element-block-name-alist' at each reload

* lisp/org-element.el (org-element-block-name-alist): Do not reset
`org-element-block-name-alist' at each reload.

lisp/org-element.el

@@ -214,7 +214,7 @@ regexp matching one object can also match the other object.")
 	 table-cell underline)
   "List of recursive object types.")
 
-(defconst org-element-block-name-alist
+(defvar org-element-block-name-alist
   '(("CENTER" . org-element-center-block-parser)
     ("COMMENT" . org-element-comment-block-parser)
     ("EXAMPLE" . org-element-example-block-parser)