gitignore: Add venv directory to gitignore

The python venv module is the standard way of creating virtual python
environments. 'venv' is a commonly used name for the corresponding
directory. For example in the tuning docs we propose to execute 'python
-m venv venv' to setup a local virtual environment.  During development
I often have these scattered around in the source tree and they show up
as untracked files in git. Add venv to .gitignore to prevent that.

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Acked-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

.gitignore

@@ -6,3 +6,4 @@
 *.patch
 *.pyc
 __pycache__/
+venv/