pkgconfig: add girdir and typelibdir variables to .pc files

pkgconfig: add girdir and typelibdir variables to .pc files

So that the -base libs can figure out the right include paths for the
gobject-introspection tools even if core got installed into a prefix
that's not the same prefix as gobject-introspection is installed in
or it's being build in an uninstalled gstreamer setup.