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/api/proxy.h b/api/proxy.h
index 6627aac..3e76ee7 100644
--- a/api/proxy.h
+++ b/api/proxy.h
@@ -61,6 +61,7 @@
 #include "rtc_base/message_handler.h"
 #include "rtc_base/message_queue.h"
 #include "rtc_base/ref_counted_object.h"
+#include "rtc_base/system/rtc_export.h"
 #include "rtc_base/thread.h"
 
 namespace rtc {
@@ -140,8 +141,8 @@
 
 namespace internal {
 
-class SynchronousMethodCall : public rtc::MessageData,
-                              public rtc::MessageHandler {
+class RTC_EXPORT SynchronousMethodCall : public rtc::MessageData,
+                                         public rtc::MessageHandler {
  public:
   explicit SynchronousMethodCall(rtc::MessageHandler* proxy);
   ~SynchronousMethodCall() override;