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}
diff --git a/sdk/objc/api/peerconnection/RTCRtpReceiver.h b/sdk/objc/api/peerconnection/RTCRtpReceiver.h
index 9023479..7a7dace 100644
--- a/sdk/objc/api/peerconnection/RTCRtpReceiver.h
+++ b/sdk/objc/api/peerconnection/RTCRtpReceiver.h
@@ -25,7 +25,7 @@
 
 @class RTCRtpReceiver;
 
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @protocol RTCRtpReceiverDelegate <NSObject>
 
 /** Called when the first RTP packet is received.
@@ -45,7 +45,7 @@
 
 @end
 
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @protocol RTCRtpReceiver <NSObject>
 
 /** A unique identifier for this receiver. */
@@ -72,7 +72,7 @@
 
 @end
 
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @interface RTCRtpReceiver : NSObject <RTCRtpReceiver>
 
 - (instancetype)init NS_UNAVAILABLE;