cmake: link librbd_test against Boost::thread

cmake: link librbd_test against Boost::thread

otherwise we will have:

[ 83%] Building CXX object src/tools/rbd/CMakeFiles/rbd.dir/Utils.cc.o
../../../lib/librbd_test.a(test_ImageWatcher.cc.o): In function
`boost::thread::do_try_join_until(timespec const&)':
/home/kchai/ceph/build/boost/include/boost/thread/detail/thread.hpp:789:
undefined reference to
`boost::thread::do_try_join_until_noexcept(timespec const&, bool&)'
collect2: error: ld returned 1 exit status
src/test/librbd/CMakeFiles/ceph_test_librbd.dir/build.make:137: recipe
for target 'bin/ceph_test_librbd' failed
make[2]: *** [bin/ceph_test_librbd] Error 1

Signed-off-by: Kefu Chai <kchai@redhat.com>