gc.h: move rb_objspace_garbage_object_p to internal/gc.h

... to allow class.c to use the function

class.c

@@ -29,7 +29,6 @@
 #include "internal/variable.h"
 #include "ruby/st.h"
 #include "vm_core.h"
-#include "gc.h"
 
 #define id_attached id__attached__
 

gc.h

@@ -127,7 +127,6 @@ void rb_objspace_reachable_objects_from_root(void (func)(const char *category, V
 int rb_objspace_markable_object_p(VALUE obj);
 int rb_objspace_internal_object_p(VALUE obj);
 int rb_objspace_marked_object_p(VALUE obj);
-int rb_objspace_garbage_object_p(VALUE obj);
 
 void rb_objspace_each_objects(
     int (*callback)(void *start, void *end, size_t stride, void *data),

internal/gc.h

@@ -99,6 +99,7 @@ VALUE rb_class_allocate_instance(VALUE klass);
 void rb_gc_ractor_newobj_cache_clear(rb_ractor_newobj_cache_t *newobj_cache);
 size_t rb_gc_obj_slot_size(VALUE obj);
 bool rb_gc_size_allocatable_p(size_t size);
+int rb_objspace_garbage_object_p(VALUE obj);
 
 RUBY_SYMBOL_EXPORT_BEGIN
 /* gc.c (export) */