Debian: make sure osmo-bsc_mgcp ends up in its own package

... rather than being in the osmo-mgw package, which is clearly wrong.

Change-Id: Ib5d19f56b611722101040639db615184fa223f82
diff --git a/debian/osmo-mgw.install b/debian/osmo-mgw.install
index e772481..5dac968 100644
--- a/debian/osmo-mgw.install
+++ b/debian/osmo-mgw.install
@@ -1 +1 @@
-usr/bin
+usr/bin/osmo-mgw