wireguard: selftests: always call kernel makefile

These selftests are used for much more extensive changes than just the
wireguard source files. So always call the kernel's build file, which
will do something or nothing after checking the whole tree, per usual.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>