rsvg_internals: remove unused extern crate declaration

also add a lint to warn about unused things
7 jobs for alatiera/unused in 27 minutes and 19 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #173489
fedora test

00:08:49

 
  Lint
manual #173491
allowed to fail manual
clippy
passed #173490
rustfmt

00:00:23

passed #173492
tarball

00:10:07

 
  Cross Distro
passed #173495
debian_i386:testing

00:07:29

passed #173494
debian:testing

00:06:49

passed #173493
fedora:rawhide

00:08:21