Making rtc_tools:unpack_aecdump publicly visible.

Bug: webrtc:8254
Change-Id: I0dce33dc45f108e5dcfe18e22480d533f93073c4
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/38960
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21569}

rtc_tools/BUILD.gn

@@ -371,6 +371,7 @@ if (rtc_include_tests) {
     }  # rtp_analyzer
 
     rtc_executable("unpack_aecdump") {
+      visibility = [ "*" ]
       testonly = true
       sources = [
         "unpack_aecdump/unpack.cc",