Expose frame_buffer GN target

Bug: None
Change-Id: I75068b87e95575235eb937ef73279f961d0df93e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/282322
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38571}

api/video/BUILD.gn

@@ -363,6 +363,7 @@ rtc_library("builtin_video_bitrate_allocator_factory") {
 }
 
 rtc_library("frame_buffer") {
+  visibility = [ "*" ]
   sources = [
     "frame_buffer.cc",
     "frame_buffer.h",