Allow intelligibility to compile in apm

- Added files to gyp and BUILD
- Made minor fixes to get everything to compile
    and intelligibility_proc to run
- Added comments
- Auto-reformatting

Original cl is at: https://webrtc-codereview.appspot.com/57579004/

TBR=aluebs@webrtc.org

Review URL: https://codereview.webrtc.org/1182323005.

Cr-Commit-Position: refs/heads/master@{#9454}

webrtc/modules/audio_processing/BUILD.gn

@@ -89,6 +89,10 @@ source_set("audio_processing") {
     "high_pass_filter_impl.cc",
     "high_pass_filter_impl.h",
     "include/audio_processing.h",
+    "intelligibility/intelligibility_enhancer.cc",
+    "intelligibility/intelligibility_enhancer.h",
+    "intelligibility/intelligibility_utils.cc",
+    "intelligibility/intelligibility_utils.h",
     "level_estimator_impl.cc",
     "level_estimator_impl.h",
     "noise_suppression_impl.cc",