classes/create-spdx: Collect all task dependencies

Stop filtering the runtime dependencies based on do_create_sdpx (makes
it only pick up things in DEPENDS) and instead include all task
dependencies that are not the current PN. This allows other dependency
methods to be picked up correctly, for example the dependency on the
kernel used by kernel modules.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>