Mark public ObjC APIs as explicitly visible.

webrtc/base/objc/RTCDispatcher.h

Index: webrtc/base/objc/RTCDispatcher.h
diff --git a/webrtc/base/objc/RTCDispatcher.h b/webrtc/base/objc/RTCDispatcher.h
index a9b92a661fd1f6c82b90ffff4e23e691993f693c..068544313e5102e95ad544002fa5b0622c2b2b70 100644
--- a/webrtc/base/objc/RTCDispatcher.h
+++ b/webrtc/base/objc/RTCDispatcher.h
@@ -10,6 +10,8 @@
 
 #import <Foundation/Foundation.h>
 
+#import "webrtc/base/objc/RTCMacros.h"
+
 typedef NS_ENUM(NSInteger, RTCDispatcherQueueType) {
   // Main dispatcher queue.
   RTCDispatcherTypeMain,
@@ -23,6 +25,7 @@ typedef NS_ENUM(NSInteger, RTCDispatcherQueueType) {
 /** Dispatcher that asynchronously dispatches blocks to a specific
  *  shared dispatch queue.
  */
+RTC_EXPORT
 @interface RTCDispatcher : NSObject
 
 - (instancetype)init NS_UNAVAILABLE;