Specify Protobuf version 2 in paymentrequest.proto

Github-Pull: #8742
Rebased-From: c4084c2

Author: fanquake

src/qt/paymentrequest.proto

@@ -6,6 +6,8 @@
 // https://en.bitcoin.it/wiki/Payment_Request
 //
 
+syntax = "proto2";
+
 package payments;
 option java_package = "org.bitcoin.protocols.payments";
 option java_outer_classname = "Protos";