Resolve warning.

```
FuzzTesting/Sources/FuzzAsyncMessageSequence/main.swift:27:21: warning: immutable value 'msg' was never used; consider replacing with '_' or removing it
      for try await msg in decoding {
                    ^~~
                    _
```

FuzzTesting/Sources/FuzzAsyncMessageSequence/main.swift

@@ -24,7 +24,7 @@ public func FuzzAsyncMessageSequence(_ start: UnsafeRawPointer, _ count: Int) ->
   let semaphore = DispatchSemaphore(value: 0)
   Task {
     do {
-      for try await msg in decoding {
+      for try await _ in decoding {
         // TODO: Test serialization for completeness.
         // We could serialize individual messages like this:
         // let _: [UInt8] = try! msg.serializedBytes()