[rtp_rtcp] fixed lint errors in rtp_rtcp module that are not fixed in other CLs

webrtc/modules/rtp_rtcp/source/rtp_packet_history.h

Index: webrtc/modules/rtp_rtcp/source/rtp_packet_history.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_packet_history.h b/webrtc/modules/rtp_rtcp/source/rtp_packet_history.h
index 3d4c09ab95edd52006c367c960e225f86a3c4818..8e1a732b1994981b2cc2c33f16cc8d4ff41019ee 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_packet_history.h
+++ b/webrtc/modules/rtp_rtcp/source/rtp_packet_history.h
@@ -29,7 +29,7 @@ static const size_t kMaxHistoryCapacity = 9600;
 
 class RTPPacketHistory {
  public:
-  RTPPacketHistory(Clock* clock);
+  explicit RTPPacketHistory(Clock* clock);
   ~RTPPacketHistory();
 
   void SetStorePacketsStatus(bool enable, uint16_t number_to_store);