[ruby/etc] Use the correct pattern for the fallback of rb_io_descriptor()

* See d1d9aef45c

https://github.com/ruby/etc/commit/5185685003

ext/etc/etc.c

@@ -68,11 +68,12 @@ void rb_deprecate_constant(VALUE mod, const char *name);
 
 #ifndef HAVE_RB_IO_DESCRIPTOR
 static int
-rb_io_descriptor(VALUE io) {
+io_descriptor_fallback(VALUE io) {
     rb_io_t *fptr;
     GetOpenFile(io, fptr);
     return fptr->fd;
 }
+#define rb_io_descriptor io_descriptor_fallback
 #endif
 
 #ifdef HAVE_RUBY_ATOMIC_H