Docs/DocBook: Add .db files to .gitignore

These files were added with the XML crossreference patch; they make git
complain.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DocBook/.gitignore

@@ -12,5 +12,6 @@
 *.gif
 *.svg
 *.proc
+*.db
 media-indices.tmpl
 media-entities.tmpl