Predeclare PrecomputedTransactionData as sturct

Author: sipa

src/main.h

@@ -38,8 +38,8 @@ class CScriptCheck;
 class CTxMemPool;
 class CValidationInterface;
 class CValidationState;
-class PrecomputedTransactionData;
 
+struct PrecomputedTransactionData;
 struct CNodeStateStats;
 struct LockPoints;