sdk:metal_objc should be publicly visible.

TBR=kthelgason@webrtc.org,phoglund@webrtc.org

Bug: webrtc:8855
Change-Id: I1a1fdb57d908b479bac1870c4709ebfbeedc1565
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/54305
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22058}
diff --git a/sdk/BUILD.gn b/sdk/BUILD.gn
index 5acea5d..1ae826e 100644
--- a/sdk/BUILD.gn
+++ b/sdk/BUILD.gn
@@ -259,6 +259,7 @@
 
     if (rtc_use_metal_rendering) {
       rtc_static_library("metal_objc") {
+        visibility = [ "*" ]
         sources = [
           "objc/Framework/Classes/Metal/RTCMTLI420Renderer.h",
           "objc/Framework/Classes/Metal/RTCMTLI420Renderer.mm",