Update visibility on rtc_base:net_helper target

Some of the new targets are otherwise not visible by external users.

Bug: webrtc:9838
Change-Id: Idc585f45eeeb937802d5a898be57cf3d887fe142
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/290730
Auto-Submit: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39059}

rtc_base/BUILD.gn

@@ -1382,6 +1382,7 @@ rtc_library("socket_address_pair") {
 }
 
 rtc_library("net_helper") {
+  visibility = [ "*" ]
   sources = [
     "net_helper.cc",
     "net_helper.h",