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/renderer/opengl/RTCVideoViewShading.h b/sdk/objc/components/renderer/opengl/RTCVideoViewShading.h
index a570129..6876cc3 100644
--- a/sdk/objc/components/renderer/opengl/RTCVideoViewShading.h
+++ b/sdk/objc/components/renderer/opengl/RTCVideoViewShading.h
@@ -18,7 +18,7 @@
  * RTCVideoViewShading provides a way for apps to customize the OpenGL(ES) shaders used in
  * rendering for the RTCEAGLVideoView/RTCNSGLVideoView.
  */
-RTC_EXPORT
+RTC_OBJC_EXPORT
 @protocol RTCVideoViewShading <NSObject>
 
 /** Callback for I420 frames. Each plane is given as a texture. */