[!754] add missing header to CMake install files

Merge branch 'add_missing_install_header' into 'master'

See merge request [!754]

  [!754]: Nonecore/dune-common/merge_requests/754

dune/common/std/CMakeLists.txt

 install(
   FILES
   apply.hh
+  functional.hh
   make_array.hh
   memory.hh
   optional.hh