docs: scripts/sphinx-pre-install: suggest latexmk for building pdf

The usage of latexmk improves the PDF output, as it re-run
xelatex when it detects the need, in order to properly generate
indexes and cross-references.

As this is not a mandatory requirement, only suggest its
addition.
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

scripts/sphinx-pre-install

@@ -532,6 +532,7 @@ sub check_needs()
 	check_program("dot", 1);
 	check_program("convert", 1);
 	check_program("rsvg-convert", 1) if ($pdf);
+	check_program("latexmk", 1) if ($pdf);
 
 	check_distros();