COMP: fix header install path

Fix header installation path so that the headers get copied to the correct subfolder vtk/meshing/netgen. Addresses second part of issue #3 here.

The fix follows the same format as session/vtk/aeva/ext as seen here: https://gitlab.kitware.com/aeva/session/-/blob/c775178c40cea5bdf1f5baf35ed1d0eb2ac10c3f/vtk/aeva/ext/CMakeLists.txt