Add default generic arg value to Listener type

The changes introduced by dcfda58 may break existing consumers
of our typings. This commit fixes this problem by adding a default
value to the recently introduced generic argument of `Listener` type.

Signed-off-by: Miroslav Bajtoš <mbajtoss@gmail.com>

types/observer-mixin.d.ts

@@ -19,7 +19,9 @@ export interface OperationHookContext<T extends typeof ModelBase> {
   [property: string]: any;
 }
 
-export type Listener<Ctx> = (ctx: Ctx, next: (err?: any) => void) => void;
+export type Listener<Ctx = OperationHookContext<typeof ModelBase>> = (
+  ctx: Ctx, next: (err?: any) => void
+) => void;
 
 export interface ObserverMixin {
   /**