Enable descriminator for kotlin-client multiplatform

Author: yshrsmz

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinClientCodegen.java

@@ -966,7 +966,7 @@ public ModelsMap postProcessModels(ModelsMap objs) {
     @Override
     public Map<String, ModelsMap> postProcessAllModels(Map<String, ModelsMap> objs) {
         objs = super.postProcessAllModels(objs);
-        if (getSerializationLibrary() == SERIALIZATION_LIBRARY_TYPE.kotlinx_serialization) {
+        if (getSerializationLibrary() == SERIALIZATION_LIBRARY_TYPE.kotlinx_serialization || getLibrary().equals(MULTIPLATFORM)) {
             // The loop removes unneeded variables so commas are handled correctly in the related templates
             for (Map.Entry<String, ModelsMap> modelsMap : objs.entrySet()) {
                 for (ModelMap mo : modelsMap.getValue().getModels()) {