[!148] Fix installation of ddd.h

Merge branch 'cherrypick/install-ddd.h' into 'releases/2.7'

See merge request [!148]

  [!148]: Nonestaging/dune-uggrid/merge_requests/148

dune/uggrid/parallel/ddd/CMakeLists.txt

+add_subdirectory(include)
+
 install(FILES
   dddconstants.hh
   dddcontext.hh
   dddtypes.hh
   dddtypes_impl.hh
-  include/ddd.h
   DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/uggrid/parallel/ddd)
 
 if(UG_ENABLE_PARALLEL)

dune/uggrid/parallel/ddd/include/CMakeLists.txt

+install(FILES
+  ddd.h
+  DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/uggrid/parallel/ddd/include)