sstate: Preserve permissions when extracting tar archive

This is done by default when tar is run by the superuser, but for native
recipes the corresponding task is not run as root under pseudo, so pass
the flag explicitly.

Suggested-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Jacob Kroon <jacob.kroon@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>