sstate.bbclass: consider postinstall dependencies too

When checking the dependencies in setscene_depvalid(), make sure we also
consider those dependencies needed when running the postinstalls on
host.

[YOCTO #3918]

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>