Restrict RNN VAD and PFFFT wrapper visibility

Bug: webrtc:10482
Change-Id: Idb0f8a87ef881970b51fcfe3296fef4924094c81
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/129860
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27305}

modules/audio_processing/agc2/rnn_vad/BUILD.gn

@@ -9,6 +9,7 @@
 import("../../../../webrtc.gni")
 
 rtc_source_set("rnn_vad") {
+  visibility = [ "../*" ]
   sources = [
     "common.h",
     "features_extraction.cc",

modules/audio_processing/utility/BUILD.gn

@@ -84,6 +84,7 @@ rtc_source_set("ooura_fft") {
 }
 
 rtc_source_set("pffft_wrapper") {
+  visibility = [ "../*" ]
   sources = [
     "pffft_wrapper.cc",
     "pffft_wrapper.h",