Add missing ${misc:Built-Using} to Built-Using on postfix-policyd-proxy.

Changes-By: lintian-brush
Fixes: lintian: missing-built-using-field-for-golang-package
See-also: https://lintian.debian.org/tags/missing-built-using-field-for-golang-package.html

debian/control

@@ -9,5 +9,6 @@ XS-Go-Import-Path: git.autistici.org/ai3/tools/postfix-policyd-proxy
 Package: postfix-policyd-proxy
 Architecture: any
 Depends: ${shlibs:Depends}, ${misc:Depends}
+Built-Using: ${misc:Built-Using}
 Description: Postfix policyd proxy
  Proxy for postfix policy services, directed by a LDAP backend.