services: configuration: Export 'no-serialization' syntactic keyword.

This is a followup to 3f9a12dc08.

* gnu/services/configuration.scm (no-serialization): New variable.

gnu/services/configuration.scm

@@ -44,6 +44,7 @@
             configuration-error?
 
             define-configuration
+            no-serialization
 
             serialize-configuration
             define-maybe
@@ -190,6 +191,9 @@ does not have a default value" field kind)))
     				       #,(id #'stem #'stem #'-fields))
     	       conf)))))))
 
+(define no-serialization         ;syntactic keyword for 'define-configuration'
+  '(no serialization))
+
 (define-syntax define-configuration
   (lambda (s)
     (syntax-case s (no-serialization)