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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/Makefile

@@ -26,7 +26,7 @@ SPHINX_CONF   = conf.py
 PAPER         =
 BUILDDIR      = $(obj)/output
 PDFLATEX      = xelatex
-LATEXOPTS     = -interaction=batchmode
+LATEXOPTS     = -interaction=batchmode -no-shell-escape
 
 ifeq ($(KBUILD_VERBOSE),0)
 SPHINXOPTS    += "-q"