Add public visibility for scalability_mode target

Bug: webrtc:11607
Change-Id: I93681ef6ff5918bd3bb26b006a9db9cab1805df2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/265862
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37223}

api/video_codecs/BUILD.gn

@@ -13,6 +13,7 @@ if (is_android) {
 }
 
 rtc_source_set("scalability_mode") {
+  visibility = [ "*" ]
   sources = [ "scalability_mode.h" ]
 }