Add ARPHRD_CAN, ARPHRD_MCTP to net/if_arp.h

Add the constant ARPHRD_MCTP, from Linux 5.15, to net/if_arp.h, along
with ARPHRD_CAN which was added to Linux in version 2.6.25 (commit
cd05acfe65ed2cf2db683fa9a6adb8d35635263b, "[CAN]: Allocate protocol
numbers for PF_CAN") but apparently missed for glibc at the time.

Tested for x86_64.

(cherry picked from commit a94d9659)