Compile sha1.c when ether support is included

Compile sha1.c when ether support is included

sha1 is used by ether_gen_addr after r346324. Perhaps in an ideal world we
could detect that the kernel's been compiled without sha1_* bits included
and silently fallback to arc4random instead because these platforms/kernel
configs are far and few between. It's fairly lightweight, though, so just
include it for now.