debian: added dpkg-dev build dependency.

This should have been done in c21ab5327309.

debian/control

diff --git a/debian/control b/debian/control
index 45c4fb7..5ad76cf 100644
--- a/debian/control
+++ b/debian/control
@@ -3,6 +3,7 @@
 Priority: optional
 Maintainer: Sergey Budnevitch <sb@nginx.com>
 Build-Depends: debhelper (>= 9~),
+               dpkg-dev (>= 1.16.1~),
                libgd2-xpm-dev,
                libgeoip-dev,
                libpcre3-dev,