staging: Mark deploy an sstate task

Deploy is a tricky task as it isn't in SSTATETASKS as it isn't always
present. We do need to ensure dependency chains around do_deploy tasks
are correctly handled as sstate tasks though. For now add to the list of
sstate tasks manually (like the other locale task reference).

Without this, missing manifest files could be reported now that do_deploy
tasks no longer have their dependencies added by the depvalid function.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>