Prepare for breakout of session_description.{h,cc}

External dependencies require "visibility = [ "*" ]"
to depend on the target.

Bug: webrtc:11967
Change-Id: I3df76e6c80380a8ab1f2bb6850a31a0d9065304b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/221901
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34264}

pc/BUILD.gn

@@ -185,6 +185,7 @@ rtc_library("rtc_pc_base") {
 }
 
 rtc_source_set("session_description") {
+  visibility = [ "*" ]
   sources = [
     # Future home of
     #"session_description.cc",
@@ -209,6 +210,7 @@ rtc_source_set("session_description") {
 }
 
 rtc_source_set("simulcast_description") {
+  visibility = [ "*" ]
   sources = [
     # Future home of
     # "simulcast_description.cc",