header: use getpagesize() for MNL_SOCKET_BUFFER_SIZE
header: use getpagesize() for MNL_SOCKET_BUFFER_SIZE

This system call returns PAGE_SIZE which depends on the
architecture. See linux/netlink.h for more information
on the appropriate datagram size for netlink.

Signed-off-by: Pablo Neira Ayuso <707d14912bb250caf67dfe0ea4035681fbfc4f56@netfilter.org>

Distrotech/libmnl,cyclops8456/libmnl,greearb/libmnl-ct,cyclops8456/libmnl,greearb/libmnl-ct,fqrouter/libmnl,Distrotech/libmnl,vyatta/libmnl,threatstack/libmnl,chamaken/libmnl,cyclops8456/libmnl,vyos/libmnl,chamaken/libmnl,vyos/libmnl,vyatta/libmnl,fqrouter/libmnl,threatstack/libmnl,remfalc/vyt-libmnl,remfalc/vyt-libmnl,vyos/libmnl