Add workspace resolver to fix a build warning

Without explicit resolver `cargo` will use `resolver = "1"` and print
the following warning:

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

Signed-off-by: Wiktor Kwapisiewicz <wiktor@metacode.biz>

Author: wiktor-k

Cargo.toml

@@ -1,2 +1,3 @@
 [workspace]
 members = ["cryptoki", "cryptoki-sys"]
+resolver = "2"