Fixed to not expose internal function

wrt #937

Author: k163377

src/main/kotlin/com/fasterxml/jackson/module/kotlin/Extensions.kt

@@ -53,10 +53,8 @@ fun ObjectMapper.registerKotlinModule(initializer: KotlinModule.Builder.() -> Un
 
 inline fun <reified T> jacksonTypeRef(): TypeReference<T> = object: TypeReference<T>() {}
 
-/**
- * It is public due to Kotlin restrictions, but should not be used externally.
- */
-inline fun <reified T> Any?.checkTypeMismatch(): T {
+@PublishedApi
+internal inline fun <reified T> Any?.checkTypeMismatch(): T {
     // Basically, this check assumes that T is non-null and the value is null.
     // Since this can be caused by both input or ObjectMapper implementation errors,
     // a more abstract RuntimeJsonMappingException is thrown.