cpio: respect MLPREFIX for PACKAGE_WRITE_DEPS

cpio: respect MLPREFIX for PACKAGE_WRITE_DEPS

* unlike DEPENDS/RDEPENDS the PACKAGE_WRITE_DEPS dependency isn't automatically
  extended with MLPREFIX, causing lib32-cpio to depend on 64bit ptest-runner (and
  all its deps)

* append it only for class-target to avoid nativesdk-cpio depending
  on target ptest-runner (before this patch) and depending on non-existent
  nativesdk-ptest-runner with this patch (as nativesdk.bbclass sets MLPREFIX to
  "nativesdk-")

  stdio: WARNING: Nothing PROVIDES 'nativesdk-ptest-runner'. Close matches:
  stdio: WARNING: Nothing RPROVIDES 'nativesdk-cpio' (but virtual:nativesdk:/home/pokybuild/yocto-worker/a-full/build/meta/recipes-extended/cpio/cpio_2.14.bb RDEPENDS on or otherwise requires it)
  stdio: WARNING: Nothing RPROVIDES 'nativesdk-cpio-dev' (but virtual:nativesdk:/home/pokybuild/yocto-worker/a-full/build/meta/recipes-extended/cpio/cpio_2.14.bb RDEPENDS on or otherwise requires it)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>