tty: provide tty_name() even without CONFIG_TTY

The audit subsystem just started printing the name of the tty,
but that causes a build failure when CONFIG_TTY is disabled:

kernel/built-in.o: In function `audit_log_task_info':
memremap.c:(.text+0x5e34c): undefined reference to `tty_name'
kernel/built-in.o: In function `audit_set_loginuid':
memremap.c:(.text+0x63b34): undefined reference to `tty_name'

This adds tty_name() to the list of functions that are provided
as trivial stubs in that configuration.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: db0a6fb ("audit: add tty field to LOGIN event")
Signed-off-by: Paul Moore <paul@paul-moore.com>

include/linux/tty.h

@@ -371,6 +371,7 @@ extern void proc_clear_tty(struct task_struct *p);
 extern struct tty_struct *get_current_tty(void);
 /* tty_io.c */
 extern int __init tty_init(void);
+extern const char *tty_name(const struct tty_struct *tty);
 #else
 static inline void console_init(void)
 { }
@@ -391,6 +392,8 @@ static inline struct tty_struct *get_current_tty(void)
 /* tty_io.c */
 static inline int __init tty_init(void)
 { return 0; }
+static inline const char *tty_name(const struct tty_struct *tty)
+{ return "(none)"; }
 #endif
 
 extern struct ktermios tty_std_termios;
@@ -415,7 +418,6 @@ static inline struct tty_struct *tty_kref_get(struct tty_struct *tty)
 	return tty;
 }
 
-extern const char *tty_name(const struct tty_struct *tty);
 extern const char *tty_driver_name(const struct tty_struct *tty);
 extern void tty_wait_until_sent(struct tty_struct *tty, long timeout);
 extern int __tty_check_change(struct tty_struct *tty, int sig);