Export class Arena to shared library.

This got broken in commit 6ef984af
which integrated changes from internal branch.

src/google/protobuf/arena.h

@@ -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