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 <[e-mail]>
Acked-by: Rob Landley <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

Documentation/DocBook/.gitignore

···
 *.out
 *.png
 *.gif
+*.svg
 media-indices.tmpl
 media-entities.tmpl