Define the Ack message in serialization/common.h

We're going to need this for VST2 audio processing.

src/common/serialization/common.h

@@ -41,6 +41,15 @@ static_assert(std::is_same_v<intptr_t, int64_t>);
 using native_size_t = uint64_t;
 using native_intptr_t = int64_t;
 
+/**
+ * Empty struct for when we have send a response to some operation without any
+ * result values.
+ */
+struct Ack {
+    template <typename S>
+    void serialize(S&) {}
+};
+
 /**
  * An object containing the startup options for hosting a plugin. These options
  * are passed to `yabridge-host.exe` as command line arguments, and they are