RTC_EXPORT webrtc::DesktopCapturerDifferWrapper.

This symbol is needed by Chromium, failing to export it causes [1] in the
Chromium / WebRTC component build.

[1] - https://ci.chromium.org/p/chromium/builders/try/linux-chromeos-compile-dbg/256692

Bug: webrtc:9419
Change-Id: Ic4d950f42f0cfe1db07b60f04dd06c042542612f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158122
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29594}

modules/desktop_capture/desktop_capturer_differ_wrapper.h

@@ -19,6 +19,7 @@
 #include "modules/desktop_capture/desktop_geometry.h"
 #include "modules/desktop_capture/shared_desktop_frame.h"
 #include "modules/desktop_capture/shared_memory.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
@@ -30,8 +31,9 @@ namespace webrtc {
 //
 // This class marks entire frame as updated if the frame size or frame stride
 // has been changed.
-class DesktopCapturerDifferWrapper : public DesktopCapturer,
-                                     public DesktopCapturer::Callback {
+class RTC_EXPORT DesktopCapturerDifferWrapper
+    : public DesktopCapturer,
+      public DesktopCapturer::Callback {
  public:
   // Creates a DesktopCapturerDifferWrapper with a DesktopCapturer
   // implementation, and takes its ownership.