wireguard: don't build on Linux 5.10

There are efforts underway to bring wireguard in-tree for Linux 5.4 and
to have a common build infrastructure for both 5.4 and 5.10 for
kmod-wireguard[0]. Until then, restrict kmod-wireguard to build only on
Linux 5.4, because the wireguard-compat package will not build on Linux
5.10.

[0]: https://github.com/openwrt/openwrt/pull/3885



Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>