Replace const -> constexpr for rtcp Packet Type

for consistency with other rtcp packet classes.

BUG=webrtc:5260

Review-Url: https://codereview.webrtc.org/2361853002
Cr-Commit-Position: refs/heads/master@{#14648}

webrtc/modules/rtp_rtcp/source/rtcp_packet/bye.h

@@ -24,7 +24,7 @@ class CommonHeader;
 
 class Bye : public RtcpPacket {
  public:
-  static const uint8_t kPacketType = 203;
+  static constexpr uint8_t kPacketType = 203;
 
   Bye();
   ~Bye() override {}