wallet: bdb: move TxnBegin to cpp file since it uses a bdb function

src/wallet/bdb.cpp

@@ -462,6 +462,15 @@ void BerkeleyEnvironment::ReloadDbEnv()
     Open(open_err);
 }
 
+DbTxn* BerkeleyEnvironment::TxnBegin(int flags)
+{
+    DbTxn* ptxn = nullptr;
+    int ret = dbenv->txn_begin(nullptr, &ptxn, flags);
+    if (!ptxn || ret != 0)
+        return nullptr;
+    return ptxn;
+}
+
 bool BerkeleyDatabase::Rewrite(const char* pszSkip)
 {
     while (true) {

src/wallet/bdb.h

@@ -67,14 +67,7 @@ public:
     void CloseDb(const fs::path& filename);
     void ReloadDbEnv();
 
-    DbTxn* TxnBegin(int flags)
-    {
-        DbTxn* ptxn = nullptr;
-        int ret = dbenv->txn_begin(nullptr, &ptxn, flags);
-        if (!ptxn || ret != 0)
-            return nullptr;
-        return ptxn;
-    }
+    DbTxn* TxnBegin(int flags);
 };
 
 /** Get BerkeleyEnvironment given a directory path. */