tools/docs: sphinx-build-wrapper: pdflatex is needed only for pdf

tools/docs: sphinx-build-wrapper: pdflatex is needed only for pdf

Fix the logic which checks for pdflatex. Currently, it complains
for both pdfdocs and latexdocs, but for the latter, all it is
needed is Sphinx.

Reported-by: Akira Yokosawa <akiyks@gmail.com>
Closes: https://lore.kernel.org/linux-doc/cover.1758881658.git.mchehab+huawei@kernel.org/T/#ma81ff2e11b8579e5edc23e4381e464081ae668b7
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <313df7b4aab653e8fc40c30120c0dbebf8a0bcb1.1759328070.git.mchehab+huawei@kernel.org>