Fix errors with missing IPPROTO_MH on windows

Use of IPPROTO_MH was introduced with a89f0f10de, but definition
was conditionalized to non-windows platforms.

Fix https://github.com/sam-github/libnet/commit/a4693ae62c6eaced9fcc849d87f5fd3b98eb8998#commitcomment-4273990 by @gvanem