Mark public ObjC APIs as explicitly visible.

BUG=

Review URL: https://codereview.webrtc.org/1899573002

Cr-Commit-Position: refs/heads/master@{#12431}
diff --git a/webrtc/api/objc/RTCDataChannelConfiguration.h b/webrtc/api/objc/RTCDataChannelConfiguration.h
index fbe342d..d9e7059 100644
--- a/webrtc/api/objc/RTCDataChannelConfiguration.h
+++ b/webrtc/api/objc/RTCDataChannelConfiguration.h
@@ -11,8 +11,11 @@
 #import <AvailabilityMacros.h>
 #import <Foundation/Foundation.h>
 
+#import "webrtc/base/objc/RTCMacros.h"
+
 NS_ASSUME_NONNULL_BEGIN
 
+RTC_EXPORT
 @interface RTCDataChannelConfiguration : NSObject
 
 /** Set to YES if ordered delivery is required. */