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

Add testcase for the same.

tests/message_sizes/dummy.c

@@ -0,0 +1,9 @@
+/* Just test that the file can be compiled successfully. */
+
+#include "messages2.pb.h"
+
+int main()
+{
+    return xmit_size;
+}
+