cmake: dmclock related cleanups

cmake: dmclock related cleanups

* do not include_directories() globally. use dmclock::dmclock's
  interface properties instead
* add_subdirectory(dmclock) instead of add_subdirectory(dmclock/src)
  for including its
* set dmclock_TEST instead of including the subdirectories under
  dmclock/
* objclass/class_api.cc includes PrimaryLogPG.h, which in turn includes
  indirect_intrusive_heap.h, so it'd better to compile class_api.cc as
  a part of osd.

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