This got broken in commit 6ef984af4b0c63c1c33127a12dcfc8e6359f0c9e which integrated changes from internal branch.
@@ -213,7 +213,7 @@ struct ArenaOptions {
// well as RepeatedPtrField.
#if __cplusplus >= 201103L
-class Arena final {
+class LIBPROTOBUF_EXPORT Arena final {
#else
class LIBPROTOBUF_EXPORT Arena {
#endif