libwmf: fix DEPENDS_virtclass-native

* libpng and jpeg are not expaned to native variants, causing different
  sstate checksums when libwmf-native is build with MACHINEs with
  e.g. different PACKAGE_ARCH
* maybe it's fault in handling DEPENDS_virtclass-native and it should
  expand it, but this patch fixes it too
  http://lists.linuxtogo.org/pipermail/openembedded-core/2012-September/029394.html



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