tty: define tty_open_by_driver when CONFIG_TTY is not defined

This patch adds definition of tty_open_by_driver when CONFIG_TTY is not
defined. This was supposed to have been included in commit
12e84c71b7d4ee38d51377fd494ac748ee4e6912 ("tty: export
tty_open_by_driver"). The patch follows convention for other such
functions and returns NULL.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/tty.h

@@ -422,6 +422,9 @@ static inline int __init tty_init(void)
 { return 0; }
 static inline const char *tty_name(const struct tty_struct *tty)
 { return "(none)"; }
+static inline struct tty_struct *tty_open_by_driver(dev_t device,
+		struct inode *inode, struct file *filp)
+{ return NULL; }
 #endif
 
 extern struct ktermios tty_std_termios;