netinet/if_ether.h: add ETH_P_MRP from linux v5.8

Ethernet protocol number for media redundancy protocol, see

  linux commit 4714d13791f831d253852c8b5d657270becb8b2a
  bridge: uapi: mrp: Add mrp attributes.
diff --git a/include/netinet/if_ether.h b/include/netinet/if_ether.h
index a08485e..55a2ff1 100644
--- a/include/netinet/if_ether.h
+++ b/include/netinet/if_ether.h
@@ -59,6 +59,7 @@
 #define ETH_P_PREAUTH	0x88C7
 #define ETH_P_TIPC	0x88CA
 #define ETH_P_LLDP	0x88CC
+#define ETH_P_MRP	0x88E3
 #define ETH_P_MACSEC	0x88E5
 #define ETH_P_8021AH	0x88E7
 #define ETH_P_MVRP	0x88F5