Fix public Obj-C header import.

Fixes the Cocoapod.

Bug: None
Change-Id: If3ace3d01ba111fc83ae18455e8ff1cb5fc7b476
Reviewed-on: https://webrtc-review.googlesource.com/1572
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Daniela Jovanoska Petrenko <denicija@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19863}

sdk/BUILD.gn

diff --git a/sdk/BUILD.gn b/sdk/BUILD.gn
index 086ce49..e660556 100644
--- a/sdk/BUILD.gn
+++ b/sdk/BUILD.gn
@@ -96,6 +96,8 @@
       ]
       configs += [ "..:common_objc" ]
 
+      public_configs = [ ":common_config_objc" ]
+
       deps = [
         ":common_objc",
         "../rtc_base:rtc_base_approved",