ubifs: CONFIG_UBIFS_FS_AUTHENTICATION should depend on UBIFS_FS

Instead of adding yet another dependency on UBIFS_FS, wrap the whole
block of ubifs config options in a single "if UBIFS_FS".

Fixes: d8a22773 ("ubifs: Enable authentication support")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>