ktap: Set MACHINE_ARCH because of ktap-module runtime dependency

* it's possible to exclude ktap-module dependency from siggen, but
  then we would need to exclude virtual/kernel and that doesn't work
  we would need to list all posisble providers as
  ktap->linux-yocto
  ktap->linux-foo
  ....

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>