Add missing _ to _hostEnd

@santiago.ospina happy new year!

This came up when building dune-copasi:

[ 61%] Building CXX object src/dune/copasi/model/diffusion_reaction/CMakeFiles/dune-copasi-lib-2D-P1.dir/model_single_compartment.cc.o
In file included from /home/runner/work/sme_deps/sme_deps/dune-copasi/src/dune/copasi/model/diffusion_reaction/model_single_compartment.cc:22:
In file included from /opt/smelibs/include/dune/grid/multidomaingrid/multidomaingrid.hh:28:
In file included from /opt/smelibs/include/dune/grid/multidomaingrid/subdomaintosubdomaininterfaceiterator.hh:5:
/opt/smelibs/include/dune/grid/multidomaingrid/subdomaininterfaceiterator.hh:106:20: error: no member named 'hostEnd' in 'SubDomainInterface<GridImp, GridView, HostGridView, IterationController>'; did you mean '_hostEnd'?
  106 |     _hostEnd = rhs.hostEnd;
      |                    ^
/opt/smelibs/include/dune/grid/multidomaingrid/subdomaininterfaceiterator.hh:325:16: note: '_hostEnd' declared here
  325 |   HostIterator _hostEnd;
      |                ^
1 error generated.