[K/N] cinterop: ignore CXIdxEntity_CXXClass in ObjC header

Unexpected behavior is caused by `@class` declaration in ObjC context.
This entity is not used in CPP context either.

kotlin-native/backend.native/tests/interop/objc/tests/kt48491.h

@@ -0,0 +1,5 @@
+@class KT48491_Foo;
+
+__attribute__((external_source_symbol(language="Swift", defined_in="KT48491_Bar",generated_declaration)))
+@interface KT48491_Foo
+@end