-Wundefined-var-template Dune::UGGrid<2>::heapSize_ with clang 7

e.g. here: https://gitlab.dune-project.org/core/dune-grid/-/jobs/100141:

In file included from /builds/core/dune-grid/dune/grid/io/file/dgfparser/dgfug.cc:7:
In file included from /builds/core/dune-grid/dune/grid/io/file/dgfparser/dgfug.hh:19:
/builds/core/dune-grid/dune/grid/uggrid.hh:771:7: warning: instantiation of variable 'Dune::UGGrid<2>::heapSize_' required here, but no definition is available [-Wundefined-var-template]
      heapSize_ = size;
      ^
/builds/core/dune-grid/dune/grid/io/file/dgfparser/dgfug.cc:112:22: note: in instantiation of member function 'Dune::UGGrid<2>::setDefaultHeapSize' requested here
      UGGrid< dim >::setDefaultHeapSize( gridParam.heapSize() );
                     ^
/builds/core/dune-grid/dune/grid/uggrid.hh:859:25: note: forward declaration of template entity is here
    static unsigned int heapSize_;
                        ^
/builds/core/dune-grid/dune/grid/uggrid.hh:771:7: note: add an explicit instantiation declaration to suppress this warning if 'Dune::UGGrid<2>::heapSize_' is explicitly instantiated in another translation unit
      heapSize_ = size;
      ^