[Kotlin] Do not use wildcard import

Description

ktlint cannot auto-correct wildcard import:

➜  openapi-generator git:(kotlin_post_process) ✗ /usr/local/bin/ktlint -F /Users/williamcheng/Code/openapi-generator/samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt
/Users/williamcheng/Code/openapi-generator/samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt:6:1: Wildcard import (cannot be auto-corrected)

openapi-generator version

Latest master

Command line used for generation

Steps to reproduce

 /usr/local/bin/ktlint -F /Users/williamcheng/Code/openapi-generator/samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/Serializer.kt

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/1400

Suggest a fix/enhancement

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