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: db0a6fb5

 ("audit: add tty field to LOGIN event")
Signed-off-by: Paul Moore <paul@paul-moore.com>