cargo_toml: Do not overwrite library name for unversioned system-deps
cargo_toml: Do not overwrite library name for unversioned system-deps

See: https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/565#note_709394

In the case of GL EGL/Wayland/X11 we use gstreamer-gl-1.0 for every
version except from 1.18 onwards, where these are renamed to
gstreamer-gl-{egl,wayland,x11}-1.0.
This requires us to manually change the name of the "unversioned" system
dep in Cargo.toml, without the generator overwriting it afterwards.

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