zebra/if_netlink: compile under musl-libc

zebra/if_netlink: compile under musl-libc

musl-libc is a lightweight libc used by alpine linux:

https://www.musl-libc.org/

AFAICT, this is the only change to the source needed to get
basic frr support compiling on musl.

Two changes in one patch, get ethhdr from netinet/if_ether.h
and replace the only __caddr_t I could find in the source base
with caddr_t.

Testing done:

Compiled apk packages using a docker environment (patches
coming soon) also compiled redhat and debian using a similar
docker environment (RFC patches for those changes are queued
up too)...

Issue: https://github.com/FRRouting/frr/issues/1859
Signed-off-by: Arthur Jones <arthur.jones@riverbed.com>