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}

sdk/objc/Framework/Classes/PeerConnection/RTCMetrics.mm

@@ -12,11 +12,11 @@
 
 #import "RTCMetricsSampleInfo+Private.h"
 
-void RTCEnableMetrics() {
+void RTCEnableMetrics(void) {
   webrtc::metrics::Enable();
 }
 
-NSArray<RTCMetricsSampleInfo *> *RTCGetAndResetMetrics() {
+NSArray<RTCMetricsSampleInfo *> *RTCGetAndResetMetrics(void) {
   std::map<std::string, std::unique_ptr<webrtc::metrics::SampleInfo>>
       histograms;
   webrtc::metrics::GetAndReset(&histograms);