add Map forward declaration in arena.h

Signed-off-by: DNKpp <dnkpp2011@gmail.com>

src/google/protobuf/arena.h

@@ -75,6 +75,8 @@ namespace protobuf {
 class Arena;    // defined below
 class Message;  // defined in message.h
 class MessageLite;
+template <typename Key, typename T>
+class Map;
 
 namespace arena_metrics {