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

Modules/FindHDF5.cmake

@@ -680,7 +680,7 @@ if( NOT HDF5_FOUND )
             PATH_SUFFIXES include Include
             ${_HDF5_SEARCH_OPTS}
         )
-        mark_as_advanced(HDF5_${LANGUAGE}_INCLUDE_DIR)
+        mark_as_advanced(HDF5_${__lang}_INCLUDE_DIR)
         list(APPEND HDF5_INCLUDE_DIRS ${HDF5_${__lang}_INCLUDE_DIR})
 
         # find the HDF5 libraries