psplash: use git repository

psplash doesn't see a new release for quite some time. The lattest
commits brought some nice bug fixes and a feature that allows to
easily customize the splash image.

[Peter: COPYING file no longer available]
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>