Add more build dependencies

Add python-virtualenv build dependency.

Change-Id: Ie2d779e672704887b6d5ffe24338619ee38c6ef5
diff --git a/debian/control b/debian/control
index 8d2787f..404b76c 100644
--- a/debian/control
+++ b/debian/control
@@ -2,7 +2,7 @@
 Maintainer: Jan Kaufman <jan.kaufman@tcpcloud.eu>
 Section: admin
 Priority: optional
-Build-Depends: debhelper (>= 9), salt-master, python, python-yaml
+Build-Depends: debhelper (>= 9), salt-master, python, python-yaml, python-virtualenv
 Standards-Version: 3.9.6
 Homepage: http://www.tcpcloud.eu
 Vcs-Browser: https://github.com/tcpcloud/salt-formula-tftpd-hpa