Fix iwyu error introduced recently.

This fixes a build error introduced by:
https://webrtc-review.googlesource.com/c/src/+/349001

bot:
https://ci.chromium.org/ui/p/chromium/builders/webrtc.fyi/WebRTC%20Chromium%20FYI%20ios-device

Error:

../../third_party/webrtc/api/video_codecs/libaom_av1_encoder_factory.cc:494:27: error: expected ';' after expression
494 |   rtc::SimpleStringBuilder sb(buf);
|                           ^
|                           ;
../../third_party/webrtc/api/video_codecs/libaom_av1_encoder_factory.cc:494:8: error: no member named 'SimpleStringBuilder' in namespace 'rtc'

Bug: none
Change-Id: I020040f367005ab56068cf4356e5effb380b7200
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/349320
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42189}
diff --git a/api/video_codecs/BUILD.gn b/api/video_codecs/BUILD.gn
index f994d5a..e5b34f8 100644
--- a/api/video_codecs/BUILD.gn
+++ b/api/video_codecs/BUILD.gn
@@ -374,6 +374,7 @@
     ":video_encoder_interface",
     "../../api/units:time_delta",
     "../../rtc_base:logging",
+    "../../rtc_base:stringutils",
     "//third_party/libaom",
   ]
 
diff --git a/api/video_codecs/libaom_av1_encoder_factory.cc b/api/video_codecs/libaom_av1_encoder_factory.cc
index 6cb789b..52bf5d6 100644
--- a/api/video_codecs/libaom_av1_encoder_factory.cc
+++ b/api/video_codecs/libaom_av1_encoder_factory.cc
@@ -19,6 +19,7 @@
 #include "absl/algorithm/container.h"
 #include "api/video_codecs/video_encoder_interface.h"
 #include "rtc_base/logging.h"
+#include "rtc_base/strings/string_builder.h"
 #include "third_party/libaom/source/libaom/aom/aom_codec.h"
 #include "third_party/libaom/source/libaom/aom/aom_encoder.h"
 #include "third_party/libaom/source/libaom/aom/aomcx.h"