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/RTCConfiguration.h b/webrtc/api/objc/RTCConfiguration.h
index 144c8d3..38ae88d 100644
--- a/webrtc/api/objc/RTCConfiguration.h
+++ b/webrtc/api/objc/RTCConfiguration.h
@@ -10,6 +10,8 @@
 
 #import <Foundation/Foundation.h>
 
+#import "webrtc/base/objc/RTCMacros.h"
+
 @class RTCIceServer;
 
 /**
@@ -50,6 +52,7 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+RTC_EXPORT
 @interface RTCConfiguration : NSObject
 
 /** An array of Ice Servers available to be used by ICE. */