docs: Makefile: Add -no-shell-escape option to LATEXOPTS

docs: Makefile: Add -no-shell-escape option to LATEXOPTS

It turns out that LaTeX enables \write18, which allows (some) shell
commands to be executed from the document source, by default.  This the
often-seen warning during a pdfdocs build:

  restricted \write18 enabled

That is a potential security problem and is entirely unnecessary; nothing
in the kernel PDF docs build needs that capability.  So disable \write18
explicitly.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/519bd2d9-1bee-03e1-eeb4-d9883c18be0c@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>