Change visibility on target mock_network_control

This is to allow external tests to depend on it.

Bug: none
Change-Id: Ic8e2f864041d959f673e7f2c18eb563a13274dcc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/298745
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Auto-Submit: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39646}

api/transport/BUILD.gn

@@ -155,6 +155,7 @@ if (rtc_include_tests) {
 
 if (rtc_include_tests) {
   rtc_source_set("mock_network_control") {
+    visibility = [ "*" ]
     testonly = true
     sources = [ "test/mock_network_control.h" ]
     deps = [