xfstests: fix compile error of src/fssum.c on bigendian host

Definition of htonll() is needed on bigendian host too, otherwise
src/fssum.c won't compile on ppc64/s390x hosts.

Acked-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>