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/thread.h b/rtc_base/thread.h
index 58df253..01a3917 100644
--- a/rtc_base/thread.h
+++ b/rtc_base/thread.h
@@ -27,6 +27,7 @@
 #include "rtc_base/message_queue.h"
 #include "rtc_base/platform_thread_types.h"
 #include "rtc_base/socket_server.h"
+#include "rtc_base/system/rtc_export.h"
 #include "rtc_base/thread_annotations.h"
 
 #if defined(WEBRTC_WIN)
@@ -77,7 +78,7 @@
 
 }  // namespace rtc_thread_internal
 
-class ThreadManager {
+class RTC_EXPORT ThreadManager {
  public:
   static const int kForever = -1;
 
@@ -132,7 +133,7 @@
 
 // WARNING! SUBCLASSES MUST CALL Stop() IN THEIR DESTRUCTORS!  See ~Thread().
 
-class RTC_LOCKABLE Thread : public MessageQueue {
+class RTC_EXPORT RTC_LOCKABLE Thread : public MessageQueue {
  public:
   explicit Thread(SocketServer* ss);
   explicit Thread(std::unique_ptr<SocketServer> ss);