nls: Hide new NLS_UCS2_UTILS

NLS_UCS2_UTILS is an option selected by filesystems that need it,
don't expose it to users.

Fixes: 089f7f59 ("fs/smb: Swing unicode common code from smb->NLS")
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Steve French <stfrench@microsoft.com>