package_pkgdata: Avoid task hash mismatches for generic task changes

Add SSTATETASKS to package_prepare_pkgdata[vardepsexclude] since otherwise
the task hashes vary depending upon which packaging backends are enabled
and likely other changes which add/remove unrelated sstate tasks.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 4011d31d)
Signed-off-by: Steve Sakoman <steve@sakoman.com>