Add missing LIBPROTOBUF_EXPORT.

src/google/protobuf/generated_message_util.h

@@ -112,12 +112,13 @@ class ArenaString;
 // pointer to a copy of the string that resides in *arena.  Requires both
 // args to be non-NULL.  If something goes wrong while reading the data
 // then NULL is returned (e.g., input does not start with a valid varint).
-ArenaString* ReadArenaString(::google::protobuf::io::CodedInputStream* input,
-                             ::google::protobuf::Arena* arena);
+LIBPROTOBUF_EXPORT ArenaString* ReadArenaString(
+    ::google::protobuf::io::CodedInputStream* input,
+    ::google::protobuf::Arena* arena);
 
 // Helper function to crash on merge failure.
 // Moved out of generated code to reduce binary size.
-void MergeFromFail(const char* file, int line) GOOGLE_ATTRIBUTE_NORETURN;
+LIBPROTOBUF_EXPORT void MergeFromFail(const char* file, int line) GOOGLE_ATTRIBUTE_NORETURN;
 
 }  // namespace internal
 }  // namespace protobuf