Add RTC_UNUSED for call to write() in TaskQueue libevent dtor.

TBR=terelius@webrtc.org

Change-Id: I9ef648299754f6cab30c278d6a803dbc782a2292
Bug: webrtc:8834
Reviewed-on: https://webrtc-review.googlesource.com/47601
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21885}
diff --git a/rtc_base/task_queue_libevent.cc b/rtc_base/task_queue_libevent.cc
index 2f89211..9c87206 100644
--- a/rtc_base/task_queue_libevent.cc
+++ b/rtc_base/task_queue_libevent.cc
@@ -231,7 +231,7 @@
     // PostAndReplyTask(), the reply task may or may not actually run.
     // In either case, it will be deleted.
     char message = kRunReplyTask;
-    write(reply_pipe_, &message, sizeof(message));
+    RTC_UNUSED(write(reply_pipe_, &message, sizeof(message)));
   }
 
  private: