Fix issue with Tempita recompilation
xref https://github.com/mesonbuild/meson-python/issues/589#issuecomment-1987206217 @rhshadrach I think you mentioned this originally on Slack
@lithomas1 - thought you might want to review. No objections here - I think adding the files as sources was originally a hack to workaround a bug in meson. Thanks @WillAyd  The more I have used this the less I am sure that it actually fixed the issue. Whenever I edit a tempita file now it does _not_ trigger a recompilation as it should