tty: move tty_ldisc_receive_buf to tty_flip.h

It's the only remaining tty_buffer.c prototype residing in tty.h. Move
it along others to tty_flip.h.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210723103147.18250-6-jslaby@suse.cz


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