cuirass: Properly convert list of <license> objects.

Fixes a bug whereby we would 'write' raw <license> objects when they
were in a list.

* build-aux/cuirass/gnu-system.scm (entry->sexp-entry): Add recursive
case when O is a list.

build-aux/cuirass/gnu-system.scm

@@ -44,4 +44,6 @@
     ((? license?) `((name . (license-name o))
                     (uri . ,(license-uri o))
                     (comment . ,(license-comment o))))
+    ((lst ...)
+     (map entry->sexp-entry lst))
     (_ o)))