`GString` changes cause invalid code to be generated

[sdroege]

CC @jf2048

Since #1327 the following is generated in gstreamer-rs. I didn't look at the cause yet.

error: expected expression, found `)`
   --> gstreamer-rs/gstreamer-webrtc/src/auto/web_rtc_data_channel.rs:176:88
    |
176 |             f(&from_glib_borrow(this), Option::<glib::GString>::from_glib_borrow(data(*)).as_ref().map(|s| s.as_str()))
    |                                                                                        ^ expected expression

error: expected expression, found `)`
   --> gstreamer-rs/gstreamer-webrtc/src/auto/web_rtc_data_channel.rs:219:88
    |
219 |             f(&from_glib_borrow(this), Option::<glib::GString>::from_glib_borrow(data(*)).as_ref().map(|s| s.as_str()))
    |                                                                                        ^ expected expression