exit: Stop exporting do_exit

Now that there are no more modular uses of do_exit remove the EXPORT_SYMBOL.

Suggested-by: Christoph Hellwig <[e-mail]>
Signed-off-by: "Eric W. Biederman" <[e-mail]>

kernel/exit.c

···
 	lockdep_free_task(tsk);
 	do_task_dead();
 }
-EXPORT_SYMBOL_GPL(do_exit);
 
 void __noreturn make_task_dead(int signr)
 {