ceph.spec.in: Always depend on junit4 (fixes bug #6216)

The RPM spec file currently lists `junit` as a dependency on all
platforms except recent versions of OpenSUSE. This is problematic
because `junit` refers to version 3.x, which doesn't satisfy the check
in the configure script:

    configure: Cannot find junit4.jar (apt-get install junit4)

As a result, rpmbuild doesn't build `libcephfs-test.jar` and fails:

    http://tracker.ceph.com/issues/6216

So let's depend on `junit4` in all cases. This package is provided by
all supported platforms (el6, fc17, fc18, fc19, opensuse12.2,
opensuse12, sles11), and my testing on CentOS 6 shows that rpmbuild
now succeeds and produces the expected `libcephfs_jni1` RPM.