Update features when generating rules.mk for spin crate

Bug: 339424309
Test: cargo_embargo generate cargo_embargo.json
Test: trusty/vendor/google/aosp/scripts/build.py \
      generic-x86_64-test --skip-tests
Change-Id: Iac4c359b2526507109a64e9e19a5ee586629b990
diff --git a/crates/spin/cargo_embargo.json b/crates/spin/cargo_embargo.json
index 928ccdf..4bdd1ae 100644
--- a/crates/spin/cargo_embargo.json
+++ b/crates/spin/cargo_embargo.json
@@ -32,6 +32,9 @@
       "generate_androidbp": false,
       "generate_rulesmk": true,
       "features": [
+        "once",
+        "mutex",
+        "spin_mutex"
       ],
       "package": {
         "spin": {