Check in CI that we compile if all rustls features are set

We assert that we still succeed compilation if multiple `rustls`
features are set.

Author: tnull

.github/workflows/cont_integration.yml

@@ -41,6 +41,7 @@ jobs:
       - run: cargo check --verbose --no-default-features --features=proxy,use-openssl
       - run: cargo check --verbose --no-default-features --features=proxy,use-rustls
       - run: cargo check --verbose --no-default-features --features=proxy,use-rustls-ring
+      - run: cargo check --verbose --no-default-features --features=proxy,use-rustls,use-rustls-ring
 
   fmt:
     name: Rust fmt