services: prosody: Make sure 'id' is available at expansion time.

* gnu/services/messaging.scm (id): Use 'define-syntax-rule' instead of
'define' to make sure 'id' is available at expansion time.

gnu/services/messaging.scm

@@ -49,8 +49,9 @@
 ;;;
 ;;; Code:
 
-(define (id ctx . parts)
-  (datum->syntax ctx (apply symbol-append (map syntax->datum parts))))
+(define-syntax-rule (id ctx parts ...)
+  "Assemble PARTS into a raw (unhygienic) identifier."
+  (datum->syntax ctx (symbol-append (syntax->datum parts) ...)))
 
 (define-syntax define-maybe
   (lambda (x)