doc: Add "*.svg" to DocBook/.gitignore

doc: Add "*.svg" to DocBook/.gitignore

Mauro add "*.png" and "*.gif" in DocBook/.gitignore in commit 6d172492,
but forgot to add "*.svg", this made git unnecessary to track directory
DocBook/media_api generated by `make htmldocs`:

 $ git status
 # On branch master
 # Untracked files:
 #   (use "git add <file>..." to include in what will be committed)
 #
 #	Documentation/DocBook/media_api/
 nothing added to commit but untracked files present (use "git add" to track)

Add "*.svg" to DocBook/.gitignore so git will not track directory
DocBook/media_api, since there is no file in that directory it interest in.

Signed-off-by: ZHAO Gang <gamerh2o@gmail.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>