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/base/RTCMacros.h b/sdk/objc/base/RTCMacros.h
index 4379058..c2def91 100644
--- a/sdk/objc/base/RTCMacros.h
+++ b/sdk/objc/base/RTCMacros.h
@@ -11,12 +11,15 @@
 #ifndef SDK_OBJC_BASE_RTCMACROS_H_
 #define SDK_OBJC_BASE_RTCMACROS_H_
 
-#define RTC_EXPORT __attribute__((visibility("default")))
+#define RTC_OBJC_EXPORT __attribute__((visibility("default")))
+// TODO(mbonadei): Remove RTC_EXPORT in order to be able to land
+// https://webrtc-review.googlesource.com/c/src/+/97960.
+#define RTC_EXPORT RTC_OBJC_EXPORT
 
 #if defined(__cplusplus)
-#define RTC_EXTERN extern "C" RTC_EXPORT
+#define RTC_EXTERN extern "C" RTC_OBJC_EXPORT
 #else
-#define RTC_EXTERN extern RTC_EXPORT
+#define RTC_EXTERN extern RTC_OBJC_EXPORT
 #endif
 
 #ifdef __OBJC__