Mark private struct fields in -sys bindings as public
Mark private struct fields in -sys bindings as public

This is needed for various structs in GStreamer because either
1) it's a private padding field, but the struct has to be declared
   as a constant to be useful (GstPluginDesc)
2) it's a semi-private field that in C would be accessed via a macro

This also makes the GValue type field public, and solves the following
FIXME comment: https://github.com/gtk-rs/glib/blob/master/src/value.rs#L144

GuillaumeGomez/gir,GuillaumeGomez/gir,EPashkin/gir,gtk-rs/gir,gtk-rs/gir,gtk-rs/gir,EPashkin/gir,GuillaumeGomez/gir