core-image-sato-sdk-ptest: these images need ptest

As this image isn't buildable without ptest (the packages won't exist),
depend on the ptest DISTRO_FEATURE to ensure we don't try and build it.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>