tty: tty_flip.h needs only tty_buffer and tty_port

tty: tty_flip.h needs only tty_buffer and tty_port

tty_flip.h currently includes whole tty.h. In fact, it needs only
tty_buffer and tty_port definitions. Provided, we separated tty_buffer
and tty_port into separate headers in the previous patch, we can make
tty_flip.h to include only much lighter tty_buffer.h and tty_port.h.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210723103147.18250-9-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>