debian: fix osmo-sgsn.install, tweak VCS link and descriptions

Change-Id: I6493a7ca7d996fab00d99652a74a709c860a3de7
diff --git a/debian/osmo-sgsn.install b/debian/osmo-sgsn.install
index 55086cc..a92ae18 100644
--- a/debian/osmo-sgsn.install
+++ b/debian/osmo-sgsn.install
@@ -1,4 +1,2 @@
-usr/bin
-usr/share/doc/openbsc/examples/osmo-gtphub
-usr/share/doc/openbsc/examples/osmo-gbproxy
+usr/bin/osmo-sgsn
 usr/share/doc/openbsc/examples/osmo-sgsn