Add missing cs_wallet lock that triggers new lock held assertion

A new AssertLockHeld(cs_wallet) call was added in commit a58370e
"Dedup nTimeFirstKey update logic" (part of PR #9108).

The lock held assertion will fail when loading prexisting wallets files from
before the #9108 merge that have watch-only keys.

src/wallet/walletdb.cpp

@@ -559,8 +559,8 @@ DBErrors CWalletDB::LoadWallet(CWallet* pwallet)
     bool fNoncriticalErrors = false;
     DBErrors result = DB_LOAD_OK;
 
-    try {
     LOCK(pwallet->cs_wallet);
+    try {
         int nMinVersion = 0;
         if (Read((string)"minversion", nMinVersion))
         {