refactor: CTxMemPool::UpdateChild() requires CTxMemPool::cs lock

No change in behavior, the lock is already held at call sites.

src/txmempool.cpp

@@ -968,6 +968,7 @@ void CTxMemPool::addUnchecked(const CTxMemPoolEntry &entry, bool validFeeEstimat
 
 void CTxMemPool::UpdateChild(txiter entry, txiter child, bool add)
 {
+    AssertLockHeld(cs);
     setEntries s;
     if (add && mapLinks[entry].children.insert(child).second) {
         cachedInnerUsage += memusage::IncrementalDynamicUsage(s);