library: Properly set the track primary_key from file

First this was missing, then I set it to the short path, now it's
correct.

Signed-off-by: Anna Schumaker <schumaker.anna@gmail.com>

lib/library.cpp

@@ -241,6 +241,7 @@ void library :: Track :: read(File &f)
 	title = f.getline();
 	filepath = f.getline();
 	title_lower = filter :: to_lowercase(title);
+	primary_key = library_db[library_id].root_path + "/" + filepath;
 	library_db[library_id].size++;
 }