DocBook: ignore .proc files

These are generated as part of 'make htmldocs'. If we don't ignore them,
then most of our generated subdirectories get treated as "untracked" by
git.
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/DocBook/.gitignore

@@ -11,5 +11,6 @@
 *.png
 *.gif
 *.svg
+*.proc
 media-indices.tmpl
 media-entities.tmpl