Add mutex requirement for AddToCompactExtraTransactions(…)

The vector `vExtraTxnForCompact`, which is guarded by the mutex
`cs_main`, is accessed in `AddToCompactExtraTransactions(…)`.

src/net_processing.cpp

@@ -560,7 +560,7 @@ bool GetNodeStateStats(NodeId nodeid, CNodeStateStats &stats) {
 // mapOrphanTransactions
 //
 
-void AddToCompactExtraTransactions(const CTransactionRef& tx)
+void AddToCompactExtraTransactions(const CTransactionRef& tx) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
     size_t max_extra_txn = gArgs.GetArg("-blockreconstructionextratxn", DEFAULT_BLOCK_RECONSTRUCTION_EXTRA_TXN);
     if (max_extra_txn <= 0)