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>
diff --git a/Documentation/DocBook/.gitignore b/Documentation/DocBook/.gitignore
index baa80b0..e05da3f 100644
--- a/Documentation/DocBook/.gitignore
+++ b/Documentation/DocBook/.gitignore
@@ -12,5 +12,6 @@
 *.gif
 *.svg
 *.proc
+*.db
 media-indices.tmpl
 media-entities.tmpl