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

include/linux/tty_flip.h

@@ -2,7 +2,8 @@
 #ifndef _LINUX_TTY_FLIP_H
 #define _LINUX_TTY_FLIP_H
 
-#include <linux/tty.h>
+#include <linux/tty_buffer.h>
+#include <linux/tty_port.h>
 
 struct tty_ldisc;