Add missing RTC_EXPORT for the component build.

Bug: webrtc:9419
Change-Id: I3225259fb4cc55e9820f590928795f4587f1e3cb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/153884
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29479}
diff --git a/rtc_base/message_handler.h b/rtc_base/message_handler.h
index 015255e..9836594 100644
--- a/rtc_base/message_handler.h
+++ b/rtc_base/message_handler.h
@@ -14,6 +14,7 @@
 #include <utility>
 
 #include "rtc_base/constructor_magic.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace rtc {
 
@@ -21,7 +22,7 @@
 
 // Messages get dispatched to a MessageHandler
 
-class MessageHandler {
+class RTC_EXPORT MessageHandler {
  public:
   virtual ~MessageHandler();
   virtual void OnMessage(Message* msg) = 0;