Fix generation of SetGValue method
Some types need special methods to convert a object to a gvalue.
The type system will look for an SetGValue method with an ref GLib.Value parameter and invoke it.
Manual generation is needed because this is a non-static method with the Handle being passed as the second parameter and the GLib.Value as the first. Without this fix, the genrator would generate the Handle for the first parameter and then the GLib.Value.