Only list plugins without external C dependencies as default-members of the cargo workspace
Only list plugins without external C dependencies as default-members of the cargo workspace

`cargo build` will only build the default members while `cargo build --all`
would continue to build all plugins.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/issues/167

sdroege/rsplugin,sdroege/rsplugin