Declare enabled option in mousewheel.d.ts

[lifecoderua]

Declare enabled option for the Mousewheel options, to prevent the type check errors.

Currently TS build raise a (non-blocking) error on build: error TS2353: Object literal may only specify known properties, and 'enabled' does not exist in type 'MousewheelOptions', using the following declaration:

<swiper
    :mousewheel="{
      enabled: true
    }"
  >
  ...