ibrdtnd: Add patch to exclude build of documentation using pdflatex

By default the package detects the presence of pdflatex and builds documentation automatically.
During the OpenWrt build this behavior is not acceptable. The added patch introduces a configure
option to explicitly enable the build of the documentation on request.

Signed-off-by: Johannes Morgenroth <morgenroth@ibr.cs.tu-bs.de>