tty: tty_io: Move 'tty_sysctl_init's prototype to shared space

Fixes the following W=1 kernel build warning(s):

 drivers/tty/tty_ldisc.c:883:6: warning: no previous prototype for ‘tty_sysctl_init’ [-Wmissing-prototypes]
 883 | void tty_sysctl_init(void)
 | ^~~~~~~~~~~~~~~

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Nick Holloway <alfie@dcs.warwick.ac.uk>
Cc: -- <julian@uhunix.uhcc.hawaii.edu>
Cc: Marko Kohtala <Marko.Kohtala@hut.fi>
Cc: Bill Hawes <whawes@star.net>
Cc: "C. Scott Ananian" <cananian@alumni.princeton.edu>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Andrew Morton <andrewm@uow.edu.eu>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-5-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -514,8 +514,6 @@ static const struct file_operations hung_up_tty_fops = {
 static DEFINE_SPINLOCK(redirect_lock);
 static struct file *redirect;
 
-extern void tty_sysctl_init(void);
-
 /**
  *	tty_wakeup	-	request more data
  *	@tty: terminal

include/linux/tty.h

@@ -716,6 +716,7 @@ extern int __must_check tty_ldisc_init(struct tty_struct *tty);
 extern void tty_ldisc_deinit(struct tty_struct *tty);
 extern int tty_ldisc_receive_buf(struct tty_ldisc *ld, const unsigned char *p,
 				 char *f, int count);
+extern void tty_sysctl_init(void);
 
 /* n_tty.c */
 extern void n_tty_inherit_ops(struct tty_ldisc_ops *ops);