Fixed small build compilation error

src/mobx.ts

@@ -21,7 +21,7 @@ registerGlobals();
 
 export { IAtom, Atom, BaseAtom                                } from "./core/atom";
 export { IObservable, IDepTreeNode                            } from "./core/observable";
-export { Reaction, IReactionPublic                            } from "./core/reaction";
+export { Reaction, IReactionPublic, IReactionDisposer         } from "./core/reaction";
 export { IDerivation, untracked, IDerivationState             } from "./core/derivation";
 export { useStrict, isStrictModeEnabled                       } from "./core/action";
 export { spy                                                  } from "./core/spy";