ibrdtnd: don't build (latex) documentation

Fixes:
http://autobuild.buildroot.net/results/05d/05d2d80050061e3fe1bd95a9644f4178ef73ef62



The build system builds api documentation if it finds pdflatex, which we
don't need to spend time on and which may fail if not all the pdflatex
dependencies are available, so disable it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>