[TTY]: Export proc_clear_tty() to modulea.

A use was added to the solaris syscall module which
can be built modular.
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/char/tty_io.c

@@ -3880,6 +3880,7 @@ void proc_clear_tty(struct task_struct *p)
 	p->signal->tty = NULL;
 	spin_unlock_irq(&p->sighand->siglock);
 }
+EXPORT_SYMBOL(proc_clear_tty);
 
 static void __proc_set_tty(struct task_struct *tsk, struct tty_struct *tty)
 {