Fix FTBFS with clang

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>

src/metadata/rmff/rmff.h

@@ -33,6 +33,8 @@
 
 namespace TagLib
 {
+   class AudioProperties;
+
    namespace RealMedia
    {
 #if SIZEOF_LONG == 4
@@ -249,9 +251,7 @@ namespace TagLib
       private:
          TagLib::ID3v1::Tag *m_id3tag;
       };
-      
+
-      class TagLib::AudioProperties;
-      
       class RealMediaFF
       {
       public: