Patch: do not ship generated files in tarball

[Ray Strode]

Hi,

> while working on packaging Plymouth on Mandriva cooker, I found a small
> error in plymouth Makefiles which put generated .plymouth files in
> tarball while .plymouth.in files (when available) should be in tarball
> instead. This is an issue when prefix used at make dist time is
> different than the one used when using tarball.
Thanks, pushed to master.