net: Add dummy nft_reject_ipv[46].ko files

This is a dummy file to please the OpenWrt build system which no longer
supports 4.19. Until we have found a better solution, this hack will
please the OpenWrt build system. Since the kernel or the modules
compiled from this repo never end up in the image that ends up on the
device, the effort spent on maintaining the package dependency hierarchy
for 4.19 in the OpenWrt build system is futile. If we ever go for a full
integration (with its own downsides in terms of maintenance cost), then
we have to maintain our own 4.19-based kernel-packages.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>
(cherry picked from commit ca32fc23)