generator: Fix generation of SetGValue method

Some types need special methods to convert an object to a GLib.Value.
The type system will look for a SetGValue method with a "ref GLib.Value"
parameter and invoke it.

A special case in the 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 generator would generate
the Handle for the first parameter and then the GLib.Value.

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>