kbuild: dummy-tools: Add python3

DRM_MSM_VALIDATE_XML depends on a python feature. Add a dummy python
interpreter to make it possible to configure this option with dummy
tools.

Fixes: b587f413 ("drm/msm/gen_header: allow skipping the validation")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Acked-by: Rob Clark <robin.clark@oss.qualcomm.com>
Link: https://patch.msgid.link/20260121105801.1827-1-msuchanek@suse.de


[nathan: Remove empty shell comment line]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>