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

src/common/serialization/vst3/base.h

@@ -69,15 +69,6 @@ std::u16string tchar_pointer_to_u16string(const Steinberg::Vst::TChar* string,
 const Steinberg::Vst::TChar* u16string_to_tchar_pointer(
     const std::u16string& string) noexcept;
 
-/**
- * Empty struct for when we have send a response to some operation without any
- * result values.
- */
-struct Ack {
-    template <typename S>
-    void serialize(S&) {}
-};
-
 /**
  * Both `TUID` (`int8_t[16]`) and `FIDString` (`char*`) are hard to work with
  * because you can't just copy them. So when serializing/deserializing them