netlink: fix bogus alignment warnings

netlink: fix bogus alignment warnings

The fix is stolen from XORP: we insert an intermediate cast to
`(void*)` to avoid the warning. Netlink stuff is already taking care
of the alignment issue, the compiler just doesn't know. This means
that the headers we copied from the kernel should be modified with:

     sed -i -e 's/\*)(((char\*/*)(void*)(((char*/g' \
            -e 's/(void\*)(void\*)/(void*)/g' include/**/*.h