Don't treat songs with different album as duplicates

src/playlist/playlist.cpp

@@ -1860,6 +1860,8 @@ void Playlist::RemoveDeletedSongs() {
 
 }
 
+namespace {
+
 struct SongSimilarHash {
   long operator() (const Song &song) const {
     return HashSimilar(song);
@@ -1872,6 +1874,8 @@ struct SongSimilarEqual {
   }
 };
 
+}  // namespace
+
 void Playlist::RemoveDuplicateSongs() {
 
   QList<int> rows_to_remove;