fix org-check-version undefined error when compiled
Prior to this change compiling org-mode and then running Org-mode
functions from a batch Emacs would throw the following error.
Symbol's function definition is void: org-check-version
Moving the require of org-compat before the use of this function in
org.el fixes this problem.
* lisp/org.el (org-compat): Require org-compat before we first use one
of its functions (a macro actually).