Add missing header file to WebRTC iOS SDK.
Add missing header file to WebRTC iOS SDK.

This header is missing from the public headers when building the
framework.

Bug: None
Change-Id: I7ce3a57d5fa34d344239dfddcc6e29aee35a2ded
Reviewed-on: https://webrtc-review.googlesource.com/79942
Commit-Queue: Anders Carlsson <a12219598ede40c00ea3b35a31f4bcac3a0c35e1@webrtc.org>
Commit-Queue: Kári Helgason <79949f6d84ac41ff1f64c59fd5a554be5ff12a02@webrtc.org>
Reviewed-by: Kári Helgason <79949f6d84ac41ff1f64c59fd5a554be5ff12a02@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#23453}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc