[ruby/readline-ext] Include ruby/assert.h in ruby/ruby.h so that assertions can be there

https://github.com/ruby/readline-ext/commit/4d44c12832

ext/readline/depend

@@ -1,6 +1,7 @@
 # AUTOGENERATED DEPENDENCIES START
 readline.o: $(RUBY_EXTCONF_H)
 readline.o: $(arch_hdrdir)/ruby/config.h
+readline.o: $(hdrdir)/ruby.h
 readline.o: $(hdrdir)/ruby/assert.h
 readline.o: $(hdrdir)/ruby/backward.h
 readline.o: $(hdrdir)/ruby/defines.h