Pass `OBJECT` option to `add_dune_mpi_flags`

The OBJECT option has to be passed on to add_dune_mpi_flags. Otherwise one will get an error when using the parallel version of UG with object libraries:

CMake Error at [...]/dune-common/cmake/modules/DuneMPI.cmake:62 (target_link_libraries):
  Object library target "[...]" may not link to anything.
Call Stack (most recent call first):
  [...]/dune-grid/cmake/modules/UseUG.cmake:206 (add_dune_mpi_flags)
  [...]/CMakeLists.txt:62 (add_dune_ug_flags)