debian: fix dependency to mgcp library

osmo-mgw has split the MGCP client to a separate library in
change-id I8e0b2d2a399b77086a36606f5e427271c6242df1
or commit 97df691307e48c39170ac39b2394a7095d7f0ee5.

Change-Id: I9c56c218fce2264fe0acae62caed4d9ff4bfb54c
diff --git a/debian/control b/debian/control
index 13d4806..ce51a7d 100644
--- a/debian/control
+++ b/debian/control
@@ -19,7 +19,7 @@
                libosmocore-dev,
                libosmo-sccp-dev,
                libosmo-abis-dev,
-               libosmo-legacy-mgcp-dev,
+               libosmo-mgcp-client-dev,
                libosmo-netif-dev,
                libosmo-ranap-dev
 Standards-Version: 3.9.8