On Linux 6.18 the path-based symvers filter can
yield an empty tmp file for external modules.
Add a 6.18-only fallback using modules.order
to populate Module.symvers.tmp.
Signed-off-by: Til Kaiser <mail@tk154.de>
Link: https://github.com/openwrt/openwrt/pull/21078
Signed-off-by: Robert Marko <robimarko@gmail.com>