Changes

core: guard Linux netlink headers by libmnl usage

Only include Linux-specific netlink headers when the libmnl backend
is enabled.

The code paths requiring <linux/if_link.h> and <linux/rtnetlink.h>
are entirely guarded by ENABLE_LIBMNL conditionals, so the headers should
not be included unconditionally.

This avoids pulling in Linux-only headers when the corresponding
backend is not built.

Change-Id: I0b681deb1f1e025e3ea9996f19b1de06a5feb408