Merge pull request #2421 from tsondergaard/3.1.x

Export fixed_address_empty_string
diff --git a/src/google/protobuf/generated_message_util.h b/src/google/protobuf/generated_message_util.h
index 8dc64c4..28ed495 100644
--- a/src/google/protobuf/generated_message_util.h
+++ b/src/google/protobuf/generated_message_util.h
@@ -111,7 +111,7 @@
 
 // Default empty string object. Don't use this directly. Instead, call
 // GetEmptyString() to get the reference.
-extern ExplicitlyConstructed< ::std::string> fixed_address_empty_string;
+LIBPROTOBUF_EXPORT extern ExplicitlyConstructed< ::std::string> fixed_address_empty_string;
 LIBPROTOBUF_EXPORT extern ProtobufOnceType empty_string_once_init_;
 LIBPROTOBUF_EXPORT void InitEmptyString();