package_ipk.bbclass: add PACKAGE_ADD_METADATA_IPK to sstate variables

Add PACKAGE_ADD_METADATA_IPK to IPKEXTRAVARS so it affects the sstate hash
of do_package_write_ipk.

Signed-off-by: Michael Ho <Michael.Ho@bmw.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>