Fix AudioSourceProviderGStreamer build with !ENABLE(MEDIA_STREAM)

https://bugs.webkit.org/show_bug.cgi?id=264119

This an another unreviewed stable branch commit, including the latest
changes that I've proposed in:

#19920

Source/WebCore/platform/audio/gstreamer/AudioSourceProviderGStreamer.h

@@ -40,28 +40,21 @@ typedef struct _GstAppSink GstAppSink;
 
 namespace WebCore {
 
-#if ENABLE(MEDIA_STREAM)
 class AudioSourceProviderGStreamer final : public WebAudioSourceProvider {
 public:
     static Ref<AudioSourceProviderGStreamer> create()
     {
         return adoptRef(*new AudioSourceProviderGStreamer());
     }
+
+#if ENABLE(MEDIA_STREAM)
     static Ref<AudioSourceProviderGStreamer> create(MediaStreamTrackPrivate& source)
     {
         return adoptRef(*new AudioSourceProviderGStreamer(source));
     }
     AudioSourceProviderGStreamer(MediaStreamTrackPrivate&);
-#else
-class AudioSourceProviderGStreamer : public AudioSourceProvider {
-    WTF_MAKE_FAST_ALLOCATED;
-    WTF_MAKE_NONCOPYABLE(AudioSourceProviderGStreamer);
-public:
 #endif
 
-    AudioSourceProviderGStreamer();
-    ~AudioSourceProviderGStreamer();
-
     void configureAudioBin(GstElement* audioBin, GstElement* audioSink);
 
     void provideInput(AudioBus*, size_t framesToProcess) override;
@@ -76,6 +69,9 @@ class AudioSourceProviderGStreamer : public AudioSourceProvider {
     void clearAdapters();
 
 private:
+    AudioSourceProviderGStreamer();
+    ~AudioSourceProviderGStreamer();
+
 #if ENABLE(MEDIA_STREAM)
     WeakPtr<MediaStreamTrackPrivate> m_captureSource;
     RefPtr<MediaStreamPrivate> m_streamPrivate;