Fix casing in generated Kotlin struct constructor function (#8849)

* fixes #8419

src/idl_gen_kotlin.cpp

diff --git a/src/idl_gen_kotlin.cpp b/src/idl_gen_kotlin.cpp
index 9101a91..4025761 100644
--- a/src/idl_gen_kotlin.cpp
+++ b/src/idl_gen_kotlin.cpp
@@ -480,7 +480,7 @@
       }
       if (IsStruct(field.value.type)) {
         GenStructBody(*field.value.type.struct_def, writer,
-                      (nameprefix + (field.name + "_")).c_str());
+                      (nameprefix + (namer_.Variable(field) + "_")).c_str());
       } else {
         writer.SetValue("type", GenMethod(field.value.type));
         writer.SetValue("argname", nameprefix + namer_.Variable(field));