Add optional output audio file to NetEq simulation API

Bug: webrtc:10337
Change-Id: I2e9071d4d2bd4b181d198031cf459965c9682775
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171518
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30873}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index 58b39d5..c5f3828 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -688,6 +688,7 @@
         "//third_party/abseil-cpp/absl/flags:flag",
         "//third_party/abseil-cpp/absl/flags:parse",
         "//third_party/abseil-cpp/absl/strings",
+        "//third_party/abseil-cpp/absl/types:optional",
       ]
     }
   }