FindHDF5: Restore HDF5_<LANG>_INCLUDE_DIR advanced cache entry mark

Refactoring in commit v3.6.0-rc1~72^2 (HDF5: Rework component searching
to correctly find HL for all bindings, 2016-05-12) renamed the language
loop variable used to construct the name of `HDF5_<LANG>_INCLUDE_DIR`
but forgot to update it in the `mark_as_advanced` call.  Fix it now.

Issue: #16651