[ruby/io-nonblock] Use the correct pattern for the fallback of rb_io_descriptor()

* See d1d9aef45c

https://github.com/ruby/io-nonblock/commit/dadb42422b

ext/io/nonblock/nonblock.c

@@ -19,12 +19,13 @@
 
 #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 F_GETFL