fix: defineNitroPlugin is undefined after build in nitro.mjs (#94)

Fixes issue with defineNitroPlugin being undefined when running `nuxi preview` in nuxt 4.
```
.output/server/chunks/nitro/nitro.mjs:13178
const _FAcPlHCrZY = defineNitroPlugin((nitro) => {
                    ^
ReferenceError: defineNitroPlugin is not defined
    at file:///.output/server/chunks/nitro/nitro.mjs:13178:21
```

src/module.ts

@@ -42,6 +42,7 @@ export default defineNuxtModule<ModuleOptions>({
       write: true,
       getContents() {
         return `
+          import { defineNitroPlugin } from '#imports'
           import { createCronHandler } from '${resolve('./runtime/server')}'
           ${files.map((file, index) => `import cronJob${index} from '${file.replace('.ts', '')}'`).join('\n')}