uapi/if_ether.h: prevent redefinition of struct ethhdr

Musl provides its own ethhdr struct definition. Add a guard to prevent
its definition of the appropriate musl header has already been included.

glibc does not implement this header, but when glibc will implement this
they can just define __UAPI_DEF_ETHHDR 0 to make it work with the
kernel.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>