Fixing WebRTC/Chromium FYI build.

After [2] has landed, some code started to be built on the iOS FYI
bots. Previous attempts to fix are [3] and [4].

Error:
  error: no member named 'SimpleStringBuilder' in namespace 'rtc'

[1] - https://ci.chromium.org/ui/p/chromium/builders/webrtc.fyi/WebRTC%20Chromium%20FYI%20ios-device/8155/overview
[2] - https://webrtc-review.googlesource.com/c/src/+/200981
[3] - https://webrtc-review.googlesource.com/c/src/+/202037
[4] - https://webrtc-review.googlesource.com/c/src/+/202038

TBR=landrey@webrtc.org

Bug: None
Change-Id: Ibee99b274f742acf41a837492d215ef45e5d9de4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/202240
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33012}

test/network/BUILD.gn

@@ -60,6 +60,7 @@ rtc_library("emulated_network") {
     "../../rtc_base:safe_minmax",
     "../../rtc_base:socket_address",
     "../../rtc_base:socket_server",
+    "../../rtc_base:stringutils",
     "../../rtc_base:task_queue_for_test",
     "../../rtc_base:threading",
     "../../rtc_base/synchronization:mutex",

test/network/emulated_turn_server.cc

@@ -14,6 +14,7 @@
 #include <utility>
 
 #include "api/packet_socket_factory.h"
+#include "rtc_base/strings/string_builder.h"
 
 namespace {