tokio example requires enabling a cargo feature

and also a dummy C example

examples/tokio/CARGO_FEATURES

@@ -0,0 +1 @@
+wasmtime-wasi/tokio

examples/tokio/main.c

@@ -0,0 +1,5 @@
+int main(int argc, char *argv[]) {
+    // This example is specific to integrating with Rust's tokio ecosystem, so
+    // it isnt applicable to C/C++.
+    return 0;
+}