KAPT: Kapt generates illegal stubs for private interface methods

Interface methods that were private got both the `default` and `private`
modifiers when using `jvm-default=all` which is not valid.

The stub will now just contain `private` in this case.

This fixes KT-48013.

plugins/kapt3/kapt3-compiler/testData/converter/jvmDefaultAll.txt

@@ -10,4 +10,7 @@ public abstract interface Foo {
     }
 
     public abstract void bar();
+
+    private void privateMethodWithDefault() {
+    }
 }