ubifs: auth: Add CONFIG_KEYS dependency

The new authentication support causes a build failure
when CONFIG_KEYS is disabled, so add a dependency.

fs/ubifs/auth.c: In function 'ubifs_init_authentication':
fs/ubifs/auth.c:249:16: error: implicit declaration of function 'request_key'; did you mean 'request_irq'? [-Werror=implicit-function-declaration]
  keyring_key = request_key(&key_type_logon, c->auth_key_name, NULL);

Fixes: d8a22773 ("ubifs: Enable authentication support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/Kconfig

@@ -86,6 +86,7 @@ config UBIFS_FS_SECURITY
 
 config UBIFS_FS_AUTHENTICATION
 	bool "UBIFS authentication support"
+	depends on KEYS
 	select CRYPTO_HMAC
 	help
 	  Enable authentication support for UBIFS. This feature offers protection