Include i420 buffers in Obj-C framework again.

These headers was lost in the cleanup CL for the Obj-C directories. This
puts them back in the framework headers.

Note that since the protocol and interface was split into two different
headers, and all public framework headers are put into a flat directory
structure, I had to rename the implementation files so they would not collide
in the framework header directory.

Bug: webrtc:9701
Change-Id: I42d4c1e02bdfa4e114575f527c4c42a19be8fb52
Reviewed-on: https://webrtc-review.googlesource.com/97330
Reviewed-by: Peter Hanspers <peterhanspers@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24539}
diff --git a/sdk/objc/base/RTCMutableI420Buffer.h b/sdk/objc/base/RTCMutableI420Buffer.h
index 312e28d..2eea6d0 100644
--- a/sdk/objc/base/RTCMutableI420Buffer.h
+++ b/sdk/objc/base/RTCMutableI420Buffer.h
@@ -16,6 +16,7 @@
 NS_ASSUME_NONNULL_BEGIN
 
 /** Extension of the I420 buffer with mutable data access */
+RTC_EXPORT
 @protocol RTCMutableI420Buffer <RTCI420Buffer, RTCMutableYUVPlanarBuffer>
 @end