populate_sdk_base.bbclass: respect MLPREFIX for ptest-pkgs's ptest-runner

* since "populate_sdk_base: ensure ptest-pkgs pulls in ptest-runner" change:
  https://git.openembedded.org/openembedded-core/commit/?id=ecff0642be5781f7f6cca617158b04ac9a0e85f0


  in multilib build when building lib32-foo-image it can pick wrong
  ptest-runner package if it was built in the same TMPDIR before the
  image, do_rootfs then fails to find it, if the package manager config
  doesn't have 64-bit feed enabled:

  opkg_prepare_url_for_install: Couldn't find anything to satisfy 'ptest-runner'

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>