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

Add testcase for the same.

tests/message_sizes/SConscript

@@ -0,0 +1,11 @@
+# Test the generation of message size #defines
+
+Import('env')
+
+incpath = env.Clone()
+incpath.Append(PROTOCPATH = '#message_sizes')
+
+incpath.NanopbProto("messages1")
+incpath.NanopbProto("messages2")
+
+incpath.Program(['dummy.c', 'messages1.pb.c', 'messages2.pb.c'])