output/wasapi: use forward declarations in the header

src/mixer/plugins/WasapiMixerPlugin.cxx

@@ -26,7 +26,9 @@
 #include <cmath>
 #include <optional>
 
+#include <audioclient.h>
 #include <endpointvolume.h>
+#include <mmdeviceapi.h>
 
 class WasapiMixer final : public Mixer {
 	WasapiOutput &output;

src/output/plugins/wasapi/WasapiOutputPlugin.cxx

@@ -18,6 +18,7 @@
  */
 
 #include "WasapiOutputPlugin.hxx"
+#include "output/OutputAPI.hxx"
 #include "lib/icu/Win32.hxx"
 #include "mixer/MixerList.hxx"
 #include "output/Error.hxx"
@@ -49,8 +50,10 @@
 #include <optional>
 #include <variant>
 
+#include <audioclient.h>
 #include <initguid.h>
 #include <functiondiscoverykeys_devpkey.h>
+#include <mmdeviceapi.h>
 
 namespace {
 static constexpr Domain wasapi_output_domain("wasapi_output");

src/output/plugins/wasapi/WasapiOutputPlugin.hxx

@@ -20,13 +20,11 @@
 #ifndef MPD_WASAPI_OUTPUT_PLUGIN_HXX
 #define MPD_WASAPI_OUTPUT_PLUGIN_HXX
 
-#include "output/OutputAPI.hxx"
-
-#include <audioclient.h>
-#include <mmdeviceapi.h>
-
 extern const struct AudioOutputPlugin wasapi_output_plugin;
 
+struct IMMDevice;
+struct IAudioClient;
+class AudioOutput;
 class WasapiOutput;
 
 [[gnu::pure]]