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/modules/desktop_capture/shared_memory.h b/modules/desktop_capture/shared_memory.h
index ba846da..2e2d2f3 100644
--- a/modules/desktop_capture/shared_memory.h
+++ b/modules/desktop_capture/shared_memory.h
@@ -20,6 +20,7 @@
 #include <memory>
 
 #include "rtc_base/constructor_magic.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
@@ -29,7 +30,7 @@
 // for video frames must extend this class with creation and destruction logic
 // specific for the target platform and then call
 // DesktopCapturer::SetSharedMemoryFactory().
-class SharedMemory {
+class RTC_EXPORT SharedMemory {
  public:
 #if defined(WEBRTC_WIN)
   typedef HANDLE Handle;