Fix clang compile warnings intriduced in #5681

src/txdb.h

@@ -16,7 +16,7 @@
 
 class CBlockFileInfo;
 class CBlockIndex;
-class CDiskTxPos;
+struct CDiskTxPos;
 class uint256;
 
 //! -dbcache default (MiB)

src/validationinterface.h

@@ -9,7 +9,7 @@
 #include <boost/signals2/signal.hpp>
 
 class CBlock;
-class CBlockLocator;
+struct CBlockLocator;
 class CTransaction;
 class CValidationInterface;
 class CValidationState;