add return type annotation for Jsx.addKeyProp

Author: mununki

jscomp/others/jsx.ml

@@ -38,8 +38,8 @@ type 'props component = ('props, element) componentLike
 external component : ('props, element) componentLike -> 'props component
   = "%identity"
 
-let addKeyProp (o : 't) (k : string) =
+let addKeyProp (p : 'props) (k : string) : 'props =
   (* Use Js_obj.assign, not Js.Obj.assign, otherwise the dependency will not be picked up correctly
      in the ninja file. *)
-  Obj.magic (Js_obj.assign (Obj.magic o) [%obj { key = k }])
+  Obj.magic (Js_obj.assign (Obj.magic p) [%obj { key = k }])
   [@@inline]