Docs: Include the Latex "ifthen" package

Otherwise we get PDF build failures when LaTeX refused to acknowledge the
existence of \ifthenelse

Fixes: 41cff161fe99d1c6a773becc2250a1dc3ac035ff
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
1 file changed