Makefile: drop separate libjson_spirit.la

Makefile: drop separate libjson_spirit.la

automake seems to have difficulty with the .la dependency on another .la.
Since libjson_spirit.la is only used by libcommon.la anyway, just build it
directly into that.  Sigh.

...
CXXLD libjson_spirit.la
AR libmds.a
CXXLD libcls_rbd.la
CXXLD libcls_rgw.la
CXXLD cephfs
CCLD test_ioctls
CC libcommon_la-ceph_ver.lo
CXX libcommon_la-version.lo
CXX ceph_dencoder.o
CCLD mount.ceph
CC ceph_ver.o
CXX test_libhadoopcephfs_build-version.o
CXXLD test_libhadoopcephfs_build
CXXLD libcommon.la
libtool: link: cannot find the library `libjson_spirit.la' or unhandled argument `libjson_spirit.la'

Signed-off-by: Sage Weil <sage@newdream.net>