debian: Add missing libmnl-dev dependency

Change-Id: I1157b1f0edabac16d664c2d4ad2250d3802dcc60
diff --git a/debian/control b/debian/control
index 651ecce..9439c3b 100644
--- a/debian/control
+++ b/debian/control
@@ -10,6 +10,7 @@
                libtool,
                pkg-config,
                libtalloc-dev,
+               libmnl-dev,
                liboping-dev (>= 1.9.0),
                libosmocore-dev (>= 1.0.1),
                libosmo-netif-dev (>= 0.4.0),