Revert "do not use mupdf in libk2pdfopt"

This reverts commit 24cdc3b41c1b38be7c6013376bbe32492a3b60c1.

k2pdfopt doesn't deal nicely with not linking MuPDF. It conditionally
leaves out functions it actually needs (which do not depend on MuPDF).

See https://github.com/koreader/koreader/issues/1317