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/RTCRtpParameters.h b/webrtc/api/objc/RTCRtpParameters.h
index d38e9ef..91d8108 100644
--- a/webrtc/api/objc/RTCRtpParameters.h
+++ b/webrtc/api/objc/RTCRtpParameters.h
@@ -11,9 +11,11 @@
 #import <Foundation/Foundation.h>
 
 #import "webrtc/api/objc/RTCRtpEncodingParameters.h"
+#import "webrtc/base/objc/RTCMacros.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
+RTC_EXPORT
 @interface RTCRtpParameters : NSObject
 
 /** The currently active encodings in the order of preference. */