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>

Documentation/conf.py

@@ -273,6 +273,7 @@ latex_elements = {
     'preamble': '''
 	% Adjust margins
 	\\usepackage[margin=0.5in, top=1in, bottom=1in]{geometry}
+        \\usepackage{ifthen}
 
         % Allow generate some pages in landscape
         \\usepackage{lscape}