Media Kit: Fix -Wmismatched-tags

Replace struct with class, since it is previously declared as a class
in SharedBufferList.h
Pointed by clang  [-Wmismatched-tags]

Change-Id: I3ff4e285862f31a2088a773be2967102bc8b18b6
Reviewed-on: https://review.haiku-os.org/480
Reviewed-by: Barrett17 <[email protected]>

headers/os/media/BufferGroup.h

@@ -11,7 +11,7 @@
 
 class BBuffer;
 namespace BPrivate {
-	struct SharedBufferList;
+	class SharedBufferList;
 }
 
 
@@ -55,7 +55,7 @@ private:
 			status_t		_Init();
 
 private:
-	friend struct BPrivate::SharedBufferList;
+	friend class BPrivate::SharedBufferList;
 
 			status_t		fInitError;
 			status_t		fRequestError;