Codechange: let NewGRF sounds make use of RandomAccessFile instead of the FIO slot functions

(cherry picked from commit c097bc9d7d)

src/fileio.cpp

@@ -39,6 +39,14 @@ static bool _do_scan_working_directory = true;
 extern std::string _config_file;
 extern std::string _highscore_file;
 
+/**
+ * Transition helper to get the RandomAccessFile associated with a given slot.
+ */
+RandomAccessFile *FioGetRandomAccessFile(int slot)
+{
+	return _fio_files[slot];
+}
+
 /**
  * Get position in the current file.
  * @return Position in the file.