dist fixes for input/regression/lilypond-book/

[Graham Percival]

All the source files in this dir should appear in the dist tarball.  I
tried a quick fix since it only complained about one file, but then it
complained about 8 more, so I'm guessing that none of these files are
included.

The simplest way to check is to do "make dist", then look inside the
tarball.  I'm not certain if test-lily/dist-check.py in GUB can be
easily run on the git tree.

I'm sure that the solution is a one-line or two-line fix to some build
file somewhere or other, but I don't know more than that.

Cheers,
- Graham