ext/io/wait/depend: make it work for Ruby 3.1.0-dev

Reverts ruby/io-wait@2eb3841
because it fails on "update-deps" check in the ruby/ruby CI.

https://github.com/ruby/ruby/runs/4230891140?check_suite_focus=true
```
diff --git a/ext/io/wait/depend b/ext/io/wait/depend
index 7b314b9..449e9fe 100644
--- a/ext/io/wait/depend
+++ b/ext/io/wait/depend
...
```

Maybe now it does not work on Ruby 2.6. This file must be changed for
each Ruby version. I have no good idea to fix this issue.

ext/io/wait/depend

@@ -1,8 +1,9 @@
 # AUTOGENERATED DEPENDENCIES START
+# wait.o: $(hdrdir)/ruby/assert.h # not in 2.6
 wait.o: $(RUBY_EXTCONF_H)
 wait.o: $(arch_hdrdir)/ruby/config.h
 wait.o: $(hdrdir)/ruby.h
-# wait.o: $(hdrdir)/ruby/assert.h # not in 2.6
+wait.o: $(hdrdir)/ruby/assert.h
 wait.o: $(hdrdir)/ruby/backward.h
 wait.o: $(hdrdir)/ruby/backward/2/assume.h
 wait.o: $(hdrdir)/ruby/backward/2/attributes.h