Add Unused to RetainedModuleValFlags

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -508,7 +508,7 @@ object Flags {
   /** Flags that can apply to a module val */
   final val RetainedModuleValFlags: FlagSet = RetainedModuleValAndClassFlags |
     Override | Final | Method | Implicit | Lazy |
-    Accessor | AbsOverride | Stable | Captured | Synchronized | Inline
+    Accessor | AbsOverride | Stable | Captured | Synchronized | Inline | Unused
 
   /** Flags that can apply to a module class */
   final val RetainedModuleClassFlags: FlagSet = RetainedModuleValAndClassFlags | ImplClass | Enum