Move header files from inc into src

In OCCT tarballs, many header files are duplicated in inc and src directories; the inc directory does not exist in OCCT git repository, header files are either generated from cdl files or maintained in the src repository.

We decided a long time ago to keep the ones from inc because it was simpler for us at that time, but cherry-picking OCCT commits will be easier if we change our mind and drop inc from our repository.