tty: rocket: fix incorrect forward declaration of 'rp_init()'

[ Upstream commit 423ea3255424b954947d167681b71ded1b8fca53 ]

Make the forward declaration actually match the real function
definition, something that previous versions of gcc had just ignored.

This is another patch to fix new warnings from gcc-9 before I start the
merge window pulls.  I don't want to miss legitimate new warnings just
because my system update brought a new compiler with new warnings.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>