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

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.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>