Fix visiblity of target test_feedback_generator_interface

It is a public interface and must be visible to allow tests to include the header file.

Bug: none
Change-Id: I4e6322c622f62c018b274b751e2c395eed7816e4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/346520
Reviewed-by: Jakob Ivarsson‎ <jakobi@webrtc.org>
Commit-Queue: Jakob Ivarsson‎ <jakobi@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Auto-Submit: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42027}

api/transport/BUILD.gn

@@ -123,6 +123,7 @@ if (rtc_include_tests) {
   rtc_source_set("test_feedback_generator_interface") {
     testonly = true
     sources = [ "test/feedback_generator_interface.h" ]
+    visibility = [ "*" ]
     deps = [
       ":network_control",
       "..:simulated_network_api",