AGC2 test: add missing include

Bug: webrtc:42232605
Change-Id: I8fcb66cf8ee27bf630433cdfee4a3386138cd7a7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/365521
Owners-Override: Jesus de Vicente Pena <devicentepena@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Jesus de Vicente Pena <devicentepena@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43419}
diff --git a/modules/audio_processing/agc2/adaptive_digital_gain_controller_unittest.cc b/modules/audio_processing/agc2/adaptive_digital_gain_controller_unittest.cc
index 39e175d..1647a5b 100644
--- a/modules/audio_processing/agc2/adaptive_digital_gain_controller_unittest.cc
+++ b/modules/audio_processing/agc2/adaptive_digital_gain_controller_unittest.cc
@@ -11,6 +11,7 @@
 #include "modules/audio_processing/agc2/adaptive_digital_gain_controller.h"
 
 #include <algorithm>
+#include <cmath>
 #include <memory>
 
 #include "api/audio/audio_processing.h"