Add explicit void to typedef functions

It is needed if we use the noImplicitAny switch in the tsconfig.json file.
Otherwise the typescript compiler won't compile it.
Error messages caused by lack of explicit return types:

node_modules/perfect-scrollbar/perfect-scrollbar.d.ts(16,3):
error TS7010: 'initialize', which lacks return-type annotation, implicitly has an 'any' return type.
node_modules/perfect-scrollbar/perfect-scrollbar.d.ts(17,3):
error TS7010: 'update', which lacks return-type annotation, implicitly has an 'any' return type.
node_modules/perfect-scrollbar/perfect-scrollbar.d.ts(18,3):
error TS7010: 'destroy', which lacks return-type annotation, implicitly has an 'any' return type.

perfect-scrollbar.d.ts

@@ -13,9 +13,9 @@ interface PerfectScrollbarOptions {
 }
 
 interface PerfectScrollbar {
-  initialize(container: HTMLElement, options?: PerfectScrollbarOptions);
-  update(container: HTMLElement);
-  destroy(container: HTMLElement);
+  initialize(container: HTMLElement, options?: PerfectScrollbarOptions): void;
+  update(container: HTMLElement): void;
+  destroy(container: HTMLElement): void;
 }
 
 interface JQuery {