Generate message size #defines also for messages defined in multiple files.

Add testcase for the same.

tests/message_sizes/messages2.proto

@@ -0,0 +1,8 @@
+import 'nanopb.proto';
+import 'messages1.proto';
+
+message xmit {
+   required MessageHeader header = 1;
+   required bytes data = 2 [(nanopb).max_size = 128];
+}
+