context: fix transfer annotation on gst_context_writable_structure

Hi,

I came across a glib critical log when trying to use GstContext from python. It looks like a glib transfer annotation was missing. Here is some sample python code that caused the critical log

import gi

gi.require_version("Gst", "1.0")
from gi.repository import Gst

Gst.init(None)

context = Gst.Context.new("test", False)
s = context.writable_structure()
del s
del context