Add csstosh.sh to EXTRA_DIST

Since css.h is in CLEANFILES, `make clean' will delete it and thus
cause the build to fail afterwards due to a missing csstoh.sh to
regenerate it. Fix this by adding csstoh.sh to EXTRA_DIST which will
cause it to be included in the tarball.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>