xfsprogs: fix static build problems caused by liburcu

xfsprogs: fix static build problems caused by liburcu

The liburcu library has a dependency on pthreads.  Hence, in order for
static builds of xfsprogs to work, $(LIBPTHREAD) needs to appear
*after* $(LUBURCU) in LLDLIBS.  Otherwise, static links of xfs_* will
fail due to undefined references of pthread_create, pthread_exit,
et. al.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>