mobile: fix build on iOS (#21362)

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 #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 {