syntax/parse: Fix syntax instead of quote-syntax in def-litset

fixes #2673

pkgs/racket-test/tests/stxparse/test-litset.rkt

@@ -124,3 +124,14 @@
     (check-equal? (kernel? #'define-values 4) #f)
     (check-equal? (kernel? #'foo) #f)
     (void)))
+
+;; Litsets with `...`
+
+(let ()
+  (define-literal-set lits #:phase 0 ([ooo ...]))
+  (test-case "litset, `...` ok"
+    (syntax-parse (quote-syntax ...) #:literal-sets (lits)
+      [ooo (void)]))
+  (tcerr "litset, `...` fails"
+    (syntax-parse #'ooo #:literal-sets (lits)
+      [ooo (void)])))

racket/collects/syntax/parse/private/litconv.rkt

@@ -176,7 +176,7 @@
                                                     'datum-external)
                                 ...))))
                (begin-for-syntax/once
-                (for ([x (in-list (syntax->list #'(external ...)))])
+                (for ([x (in-list (list (quote-syntax external) ...))])
                   (unless (identifier-binding x 'relphase)
                     (raise-syntax-error #f
                                         (format "literal is unbound in phase ~a~a~a"