Update API for Objective-C RTCDataChannel.

BUG=

Review URL: https://codereview.webrtc.org/1545393003

Cr-Commit-Position: refs/heads/master@{#11362}
diff --git a/webrtc/api/api.gyp b/webrtc/api/api.gyp
index 5f848d7..ea31c17 100644
--- a/webrtc/api/api.gyp
+++ b/webrtc/api/api.gyp
@@ -25,6 +25,9 @@
             'objc/RTCAudioTrack+Private.h',
             'objc/RTCAudioTrack.h',
             'objc/RTCAudioTrack.mm',
+            'objc/RTCDataChannel+Private.h',
+            'objc/RTCDataChannel.h',
+            'objc/RTCDataChannel.mm',
             'objc/RTCIceCandidate+Private.h',
             'objc/RTCIceCandidate.h',
             'objc/RTCIceCandidate.mm',