cargo: Set resolver = "2" and avoid warning

Since a few Rust versions `cargo` complains:

  warning: virtual workspace defaulting to `resolver = "1"` despite one or more workspace members being on edition 2021 which implies `resolver = "2"`

Avoid that by setting the resolver for the virtual manifest to `"2"`
explicitly.

Cargo.toml

@@ -1,4 +1,5 @@
 [workspace]
+resolver = "2"
 members = [
     "ndk",
     "ndk-sys",