wxPython: enable headers

Headers were disabled, probably because they caused a build error: the header files were being
copied to the store path of wxwidgets, but it failed with permission denied. With this commit,
the header files stay in the wxPython store path.
To point the builder of another derivation to the wxPython header files, some tweaking is
needed. E.g. for kicad: cmakeFlags = [ "-DCMAKE_CXX_FLAGS=-I${wxPython}/include/wx-3.0" ]