The devlink plugin allows updating firmware of network devices
implementing the kernel devlink interface. It is gated on libmnl,
which was hard-disabled so far. Wire the libmnl meson option to a
new FWUPD_PLUGIN_DEVLINK config symbol, enabled by default, and add
the corresponding libmnl runtime dependency to fwupd-libs.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>