Add gl and khronos-egl

khronos-egl is needed by gdk4-wayland and gdk4-x11

gl is apparently a dev dependency for khronos-egl but I don't understand how that works since gl is not listed in the debian/control for khronos-egl

This is my first contribution to Debian Rust packaging.

I am a Debian Developer so I didn't add an RFS file.