src: Cargo.toml: fix warnings on workspace

Since we'll be using edition = "2021", and we have a virtual
workspace[^1], we need to specify the manifest resolver[^2].

Otherwise, cargo emits the following warning:

    warning: virtual workspace defaulting to `resolver = "1"` despite one or more workspace members being on edition 2021 which implies `resolver = "2"`
    note: to keep the current resolver, specify `workspace.resolver = "1"` in the workspace root's manifest
    note: to use the edition 2021 resolver, specify `workspace.resolver = "2"` in the workspace root's manifest
    note: for more details see https://doc.rust-lang.org/cargo/reference/resolver.html#resolver-versions

See release note[^3] for details on how this resolver changes the way
features are selected for packages inside the workspace.

[^1]: https://doc.rust-lang.org/cargo/reference/workspaces.html#virtual-workspace
[^2]: https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html
[^3]: https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html#cargos-new-feature-resolver

src/rust/Cargo.toml

@@ -3,3 +3,4 @@ members = [
     "vlcrs-messages",
     "vlcrs-sys-generator"
 ]
+resolver = "2"