Fix two clang3.3 warnings

src/main.h

@@ -106,7 +106,7 @@ static const uint64 nMinDiskSpace = 52428800;
 class CReserveKey;
 class CCoinsDB;
 class CBlockTreeDB;
-class CDiskBlockPos;
+struct CDiskBlockPos;
 class CCoins;
 class CTxUndo;
 class CCoinsView;