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/components/audio/RTCAudioSession.h b/sdk/objc/components/audio/RTCAudioSession.h
index a8325b6..d43418b 100644
--- a/sdk/objc/components/audio/RTCAudioSession.h
+++ b/sdk/objc/components/audio/RTCAudioSession.h
@@ -27,7 +27,7 @@
 // Surfaces AVAudioSession events. WebRTC will listen directly for notifications
 // from AVAudioSession and handle them before calling these delegate methods,
 // at which point applications can perform additional processing if required.
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @protocol RTCAudioSessionDelegate <NSObject>
 
 @optional
@@ -102,7 +102,7 @@
  *  activation state has changed outside of RTCAudioSession. The current known use
  *  case of this is when CallKit activates the audio session for the application
  */
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @protocol RTCAudioSessionActivationDelegate <NSObject>
 
 /** Called when the audio session is activated outside of the app by iOS. */
@@ -120,7 +120,7 @@
  *  RTCAudioSession also coordinates activation so that the audio session is
  *  activated only once. See |setActive:error:|.
  */
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @interface RTCAudioSession : NSObject <RTCAudioSessionActivationDelegate>
 
 /** Convenience property to access the AVAudioSession singleton. Callers should
diff --git a/sdk/objc/components/audio/RTCAudioSessionConfiguration.h b/sdk/objc/components/audio/RTCAudioSessionConfiguration.h
index 3eb5e3a..9f3765d 100644
--- a/sdk/objc/components/audio/RTCAudioSessionConfiguration.h
+++ b/sdk/objc/components/audio/RTCAudioSessionConfiguration.h
@@ -22,7 +22,7 @@
 RTC_EXTERN const double kRTCAudioSessionLowComplexityIOBufferDuration;
 
 // Struct to hold configuration values.
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @interface RTCAudioSessionConfiguration : NSObject
 
 @property(nonatomic, strong) NSString *category;