node: Add chainman alias for g_chainman

src/validation.h

@@ -43,6 +43,7 @@ class CConnman;
 class CScriptCheck;
 class CBlockPolicyEstimator;
 class CTxMemPool;
+class ChainstateManager;
 class TxValidationState;
 struct ChainTxData;
 
@@ -493,9 +494,6 @@ enum class CoinsCacheSizeState
     OK = 0
 };
 
-// Defined below, but needed for `friend` usage in CChainState.
-class ChainstateManager;
-
 /**
  * CChainState stores and provides an API to update our local knowledge of the
  * current best chain.