Add missing 'explicit' specifier to GainControlImpl

Bug: None
Change-Id: I36049e54e61f15e7fed522f625f97bbfae71aed1
Reviewed-on: https://webrtc-review.googlesource.com/c/122460
Reviewed-by: Alessio Bazzica <alessiob@webrtc.org>
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26645}
diff --git a/modules/audio_processing/gain_control_impl.h b/modules/audio_processing/gain_control_impl.h
index 9412e88..9dfe0f1 100644
--- a/modules/audio_processing/gain_control_impl.h
+++ b/modules/audio_processing/gain_control_impl.h
@@ -30,7 +30,7 @@
 
 class GainControlImpl : public GainControl {
  public:
-  GainControlImpl(rtc::CriticalSection* crit_capture);
+  explicit GainControlImpl(rtc::CriticalSection* crit_capture);
   ~GainControlImpl() override;
 
   void ProcessRenderAudio(rtc::ArrayView<const int16_t> packed_render_audio);