mobile: fix build on iOS

This fixes the iOS framework build by naming the second parameter of the
Signer interface method. The name is important because it becomes part
of the objc method signature.

Fixes ethereum#21340

mobile/bind.go

@@ -32,7 +32,7 @@ import (
 // Signer is an interface defining the callback when a contract requires a
 // method to sign the transaction before submission.
 type Signer interface {
-	Sign(*Address, *Transaction) (tx *Transaction, _ error)
+	Sign(addr *Address, unsignedTx *Transaction) (tx *Transaction, _ error)
 }
 
 type MobileSigner struct {