scripts: documentation-file-ref-check: ignore hidden files

There's a warning there from a .gitignore file:

	tools/perf/.gitignore: Documentation/doc.dep

This is not really a cross-reference type of warning, so
no need to report it.

In a matter of fact, it doesn't make sense at all to even
parse hidden files, as some text editors may create such
files for their own usage.

So, just ignore everything that matches this pattern:

	/\.*

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/dd0125a931b4fecf8fab6be8aa527faa18f78e43.1634629094.git.mchehab+huawei@kernel.org


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