Add RTC_EXPORT to CurrentTaskQueueSetter

This allows for its use in test targets in Chromium Windows, which fixes
the compiled errors found in https://chromium-review.googlesource.com/c/chromium/src/+/3649679

Change-Id: I738b2eaab8eca73c40e847ede67ff5e7757ec512
Bug: None
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/262811
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36939}
diff --git a/api/task_queue/task_queue_base.h b/api/task_queue/task_queue_base.h
index b7c92f8..c3e79b7 100644
--- a/api/task_queue/task_queue_base.h
+++ b/api/task_queue/task_queue_base.h
@@ -130,7 +130,7 @@
   bool IsCurrent() const { return Current() == this; }
 
  protected:
-  class CurrentTaskQueueSetter {
+  class RTC_EXPORT CurrentTaskQueueSetter {
    public:
     explicit CurrentTaskQueueSetter(TaskQueueBase* task_queue);
     CurrentTaskQueueSetter(const CurrentTaskQueueSetter&) = delete;