Fix parsing of ocamllex in jbuild files

The long form would be rejected

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/dune_file.ml

@@ -1485,18 +1485,19 @@ module Rule = struct
     }
 
   let ocamllex_jbuild =
-    enter
+    peek_exn >>= function
-      (if_list
+    | List (_, Atom (_, _) :: _) ->
-         ~then_:(
+      enter (
-           record
-             (let%map modules = field "modules" (list string)
-              and mode = Mode.field in
-              { modules; mode }))
-         ~else_:(
         repeat string >>| fun modules ->
              { modules
              ; mode  = Standard
-           }))
+             }
+      )
+    | _ ->
+      record
+        (let%map modules = field "modules" (list string)
+         and mode = Mode.field in
+         { modules; mode })
 
   let ocamllex_dune =
     if_eos

test/blackbox-tests/test-cases/ocamllex-jbuild/run.t

@@ -1,6 +1,2 @@
   $ dune exec ./foo.exe
-  File "jbuild", line 2, characters 3-10:
+  foo
-   ((modules (foo))))
-     ^^^^^^^
-  Error: S-expression of the form (<name> <values>...) expected
-  [1]