Fix chromium-style errors in IntelligibilityEnhancer

BUG=
R=henrik.lundin@webrtc.org, peah@webrtc.org

Review URL: https://codereview.webrtc.org/2308043002 .

Cr-Commit-Position: refs/heads/master@{#14094}
diff --git a/webrtc/modules/audio_processing/intelligibility/intelligibility_utils.h b/webrtc/modules/audio_processing/intelligibility/intelligibility_utils.h
index 11b9e49..b5cc075 100644
--- a/webrtc/modules/audio_processing/intelligibility/intelligibility_utils.h
+++ b/webrtc/modules/audio_processing/intelligibility/intelligibility_utils.h
@@ -48,6 +48,8 @@
  public:
   GainApplier(size_t freqs, float relative_change_limit);
 
+  ~GainApplier();
+
   // Copy |in_block| to |out_block|, multiplied by the current set of gains,
   // and step the current set of gains towards the target set.
   void Apply(const std::complex<float>* in_block,