graphics/vkd3d: explicitly disable doxygen documentation

$ pkg install doxygen
$ make
[...]
!pdfTeX error: /usr/local/bin/pdflatex (file 8r.enc): cannot open encoding file
 ==> Fatal error occurred, no output PDF file produced!
mv: rename refman.pdf to ../vkd3d.pdf: No such file or directory
gmake[1]: *** [Makefile:2082: doc/vkd3d.pdf] Error 1

$ pkg provides 8r.enc
Name    : tex-dvipsk-5.995_2
Desc    : Convert a TeX DVI file to PostScript
Repo    : FreeBSD
Filename: usr/local/share/texmf-dist/fonts/enc/dvips/txfonts/tx8r.enc
          usr/local/share/texmf-dist/fonts/enc/dvips/context/q-8r.enc
          usr/local/share/texmf-dist/fonts/enc/dvips/base/8r.enc