package/python-flit-core: set license path

When generating legal info with a configuration enabling a package which uses
flit as setup type, we get a warning about python-flit-core license:
WARNING: python-flit-core-3.8.0: cannot save license (HOST_PYTHON_FLIT_CORE_LICENSE_FILES not defined)

Add missing variable to point to python-flit-core license file

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
[Peter: add sha256sum to .hash file]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>