Mark mock_peerconnectioninterface.h as publicly visible

According to gmock guidelines, mocks for API classes should live
in the same package which owns the API.

No-Try: True
Bug: webrtc:11642
Change-Id: Ib105a1806cc710bc4cff752b8950e981bb4bc326
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176381
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31429}

api/BUILD.gn

@@ -862,6 +862,7 @@ if (rtc_include_tests) {
   }
 
   rtc_source_set("mock_peerconnectioninterface") {
+    visibility = [ "*" ]
     testonly = true
     sources = [ "test/mock_peerconnectioninterface.h" ]