Add postgresql dev package as testonly neutron dep

The neutron functional job needs to build psycopg2 which has a
dependency on postgresql dev packages.

Change-Id: Ie5c8e2bd02e148d6638f4a4c1fbf6667c30313d5
diff --git a/files/debs/neutron b/files/debs/neutron
index 5a59b22..3f4b6d2 100644
--- a/files/debs/neutron
+++ b/files/debs/neutron
@@ -6,6 +6,7 @@
 libmysqlclient-dev  # testonly
 mysql-server #NOPRIME
 sudo
+postgresql-server-dev-all       # testonly
 python-iso8601
 python-paste
 python-routes