Debian: Include the osmo-bsc-mgcp systemd service in Debian package

Change-Id: Ife3f034ddaaba993bf27c1f9a34a1d57d1bf3e05
diff --git a/debian/osmo-bsc-mgcp.service b/debian/osmo-bsc-mgcp.service
new file mode 120000
index 0000000..eb73b7b
--- /dev/null
+++ b/debian/osmo-bsc-mgcp.service
@@ -0,0 +1 @@
+../contrib/systemd/osmo-bsc-mgcp.service
\ No newline at end of file