Some compilers may pad structures to a length that's a multiple of 2 or

Some compilers may pad structures to a length that's a multiple of 2 or
4 bytes, even though no member in the structure requires such an
alignment; don't use "sizeof (struct ether_arp), explicitly #define the
header length and use that #defined value.

We don't use "struct ether_addr" in any dissectors, so there's no need
to define it in "ether.h" - and, if compilers pad it to a multiple of 4
bytes, you can't use it anyway, as it'll be 8 bytes long, not 6 bytes
long.