decoder/OggUtil: add struct forward declarations

src/decoder/OggUtil.cxx

@@ -23,7 +23,7 @@
 
 bool
 OggFeed(ogg_sync_state &oy, struct decoder *decoder,
-	struct input_stream *input_stream, size_t size)
+	input_stream *input_stream, size_t size)
 {
 		char *buffer = ogg_sync_buffer(&oy, size);
 		if (buffer == nullptr)
@@ -40,7 +40,7 @@ OggFeed(ogg_sync_state &oy, struct decoder *decoder,
 
 bool
 OggExpectPage(ogg_sync_state &oy, ogg_page &page,
-	      struct decoder *decoder, struct input_stream *input_stream)
+	      decoder *decoder, input_stream *input_stream)
 {
 	while (true) {
 		int r = ogg_sync_pageout(&oy, &page);

src/decoder/OggUtil.hxx

@@ -26,14 +26,17 @@
 
 #include <stddef.h>
 
+struct input_stream;
+struct decoder;
+
 /**
  * Feed data from the #input_stream into the #ogg_sync_state.
  *
  * @return false on error or end-of-file
  */
 bool
-OggFeed(ogg_sync_state &oy, struct decoder *decoder,
-	struct input_stream *input_stream, size_t size);
+OggFeed(ogg_sync_state &oy, struct decoder *decoder, input_stream *is,
+	size_t size);
 
 /**
  * Feed into the #ogg_sync_state until a page gets available.  Garbage
@@ -43,6 +46,6 @@ OggFeed(ogg_sync_state &oy, struct decoder *decoder,
  */
 bool
 OggExpectPage(ogg_sync_state &oy, ogg_page &page,
-	      struct decoder *decoder, struct input_stream *input_stream);
+	      decoder *decoder, input_stream *input_stream);
 
 #endif