ttyprintk: Allow built as a module

The driver is well written to be used as a module, just the exit call
is missing.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/Kconfig

@@ -40,7 +40,7 @@ config SGI_MBCS
 source "drivers/tty/serial/Kconfig"
 
 config TTY_PRINTK
-	bool "TTY driver to output user messages via printk"
+	tristate "TTY driver to output user messages via printk"
 	depends on EXPERT && TTY
 	default n
 	---help---