ufs: fix function declaration for ufs_truncate_blocks

sparse says:

    fs/ufs/inode.c:1195:6: warning: symbol 'ufs_truncate_blocks' was not declared. Should it be static?

Note that the forward declaration in the file is already marked static.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>