Add html doc dependency on (unused) src/fltk-book.tex for doxygen.

This is not intended and could also be removed from Doxyfile, but
currently this is the best way - more changes to come...


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10361 ea41ed52-d2ee-0310-a9c1-e6b18d33e121