Minimal changes to comply with explicit CMutableTransaction -> CTranaction conversion.

This commit makes the minimal changes necessary to fix compilation once CTransaction(const CMutableTransaction &tx) is made explicit. In each case an explicit call `CTransaction(...)` was added. Shouldn't affect behaviour or performance.

src/wallet/rpcwallet.cpp

@@ -3058,7 +3058,7 @@ static UniValue fundrawtransaction(const JSONRPCRequest& request)
     FundTransaction(pwallet, tx, fee, change_position, request.params[1]);
 
     UniValue result(UniValue::VOBJ);
-    result.pushKV("hex", EncodeHexTx(tx));
+    result.pushKV("hex", EncodeHexTx(CTransaction(tx)));
     result.pushKV("fee", ValueFromAmount(fee));
     result.pushKV("changepos", change_position);