docs: automarkup.py: ignore exceptions when seeking for xrefs

When using the automarkup extension with:
	make pdfdocs

without passing an specific book, the code will raise an exception:

	  File "/devel/v4l/docs/Documentation/sphinx/automarkup.py", line 86, in auto_markup
	    node.parent.replace(node, markup_funcs(name, app, node))
	  File "/devel/v4l/docs/Documentation/sphinx/automarkup.py", line 59, in markup_funcs
	    'function', target, pxref, lit_text)
	  File "/devel/v4l/docs/sphinx_2.0/lib/python3.7/site-packages/sphinx/domains/c.py", line 308, in resolve_xref
	    contnode, target)
	  File "/devel/v4l/docs/sphinx_2.0/lib/python3.7/site-packages/sphinx/util/nodes.py", line 450, in make_refnode
	    '#' + targetid)
	  File "/devel/v4l/docs/sphinx_2.0/lib/python3.7/site-packages/sphinx/builders/latex/__init__.py", line 159, in get_relative_uri
	    return self.get_target_uri(to, typ)
	  File "/devel/v4l/docs/sphinx_2.0/lib/python3.7/site-packages/sphinx/builders/latex/__init__.py", line 152, in get_target_uri
	    raise NoUri
	sphinx.environment.NoUri

This happens because not all references will belong to a single
PDF/LaTeX document.

Better to just ignore those than breaking Sphinx build.

Fixes: d74b0d31

 ("Docs: An initial automarkup extension for sphinx")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
[jc: Narrowed the "except" and tweaked the comment]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>