gst-libs: fix up gobject-introspection some more

gst-libs: fix up gobject-introspection some more

Use new girdir and typlibdir from core .pc files, so we can figure
out the right includes to pass to the gobject-introspection tools,
whether core is installed in the same prefix as gobject-introspection
or in a different prefix or uninstalled. This also keeps us from adding
bogus paths to the includes that only work if core is uninstalled.

Also add some missing includes/pkgs where needed.