ubifs: Make xattr structures static

Fix sparse warnings from the use of "struct xattr_handler"
structures that are not exported by making them static. Fixes
the following sparse warnings:

/fs/ubifs/xattr.c:595:28: warning: symbol 'ubifs_user_xattr_handler' was not declared. Should it be static?
/fs/ubifs/xattr.c:601:28: warning: symbol 'ubifs_trusted_xattr_handler' was not declared. Should it be static?
/fs/ubifs/xattr.c:607:28: warning: symbol 'ubifs_security_xattr_handler' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>