added missing include guard to disk_io_thread.hpp

include/libtorrent/disk_io_thread.hpp

@@ -30,6 +30,9 @@ POSSIBILITY OF SUCH DAMAGE.
 
 */
 
+#ifndef TORRENT_DISK_IO_THREAD
+#define TORRENT_DISK_IO_THREAD
+
 #ifdef TORRENT_DISK_STATS
 #include <fstream>
 #endif
@@ -152,3 +155,5 @@ namespace libtorrent
 
 }
 
+#endif
+