[!547] added missing header to_unique_ptr.hh to CMakeLists.txt

Merge branch 'bugfix/add_to_unique_ptr_to_CmakeLists' into 'master'

Header was missing in CMakeLists.txt and now breaks code due to usage in
dune-grid Needs to be backported to 2.6

See merge request [!547]

  [!547]: gitlab.dune-project.org/core/dune-common/merge_requests/547

dune/common/CMakeLists.txt

@@ -103,6 +103,7 @@ install(FILES
         stdthread.hh
         streamoperators.hh
         stringutility.hh
+        to_unique_ptr.hh
         timer.hh
         tupleutility.hh
         tuplevector.hh