netlink.7: Fix alignment issue in example

netlink.7: Fix alignment issue in example

PVS-Studio reports that in

    char buf[8192];
    /* ... */
    nh = (struct nlmsghdr *) buf,

the pointer 'buf' is cast to a more strictly aligned pointer type.
This is undefined behaviour. One possible solution to make sure
that buf is correctly aligned is to declare buf as an array of
struct nlmsghdr. Other solutions include allocating the array on
the heap, use an union, or stdalign features.  With this patch,
the buffer still contains 8192 bytes.

This was raised on Stack Overflow:
https://stackoverflow.com/questions/57745580/netlink-receive-buffer-alignment

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>