Fixing warnings in public iOS SDK headers.
Building with the newly published cocoapod generated a few warnings,
which looked a bit bad.

Bug: webrtc:8831
Change-Id: I70c06930603b328e4d11c599a5b5dd77b45150c6
Reviewed-on: https://webrtc-review.googlesource.com/46163
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Peter Hanspers <peterhanspers@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21846}
diff --git a/sdk/objc/Framework/Headers/WebRTC/RTCMetrics.h b/sdk/objc/Framework/Headers/WebRTC/RTCMetrics.h
index fa2d48f..d22287e 100644
--- a/sdk/objc/Framework/Headers/WebRTC/RTCMetrics.h
+++ b/sdk/objc/Framework/Headers/WebRTC/RTCMetrics.h
@@ -17,7 +17,8 @@
  * Enables gathering of metrics (which can be fetched with
  * RTCGetAndResetMetrics). Must be called before any other call into WebRTC.
  */
-RTC_EXTERN void RTCEnableMetrics();
+RTC_EXTERN void RTCEnableMetrics(void);
 
 /** Gets and clears native histograms. */
-RTC_EXTERN NSArray<RTCMetricsSampleInfo *> *RTCGetAndResetMetrics();
+RTC_EXTERN NSArray<RTCMetricsSampleInfo *> *RTCGetAndResetMetrics(void);
+