Rename RTC_EXPORT to RTC_OBJC_EXPORT.

A new version of RTC_EXPORT will be introduced by [1] and it will be
used by WebRTC native code.

This CL renames the current RTC_EXPORT to RTC_OBJC_EXPORT in order
to avoid to mix them. It has been decided to avoid to unify them because
RTC_OBJC_EXPORT always marks symbols with default visibility, while
RTC_EXPORT will do it only when COMPONENT_BUILD is defined.

[1] - https://webrtc-review.googlesource.com/c/src/+/97960 is

Bug: webrtc:9419
Change-Id: I56a3fc6601c72d3ad6a58f9961a00e3761dfb5da
Reviewed-on: https://webrtc-review.googlesource.com/100521
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24754}

sdk/objc/api/peerconnection/RTCRtpTransceiver.h

@@ -56,7 +56,7 @@ typedef NS_ENUM(NSInteger, RTCRtpTransceiverDirection) {
  *  WebRTC specification for RTCRtpTransceiver, the JavaScript analog:
  *  https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiver
  */
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @protocol RTCRtpTransceiver <NSObject>
 
 /** Media type of the transceiver. The sender and receiver will also have this
@@ -118,7 +118,7 @@ RTC_EXPORT
 
 @end
 
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @interface RTCRtpTransceiver : NSObject <RTCRtpTransceiver>
 
 - (instancetype)init NS_UNAVAILABLE;