Add missing require

This is useful if you want to use IRB::ColorPrinter as a library like:

```
begin
  require 'irb/color_printer'
  IRB::ColorPrinter.pp(obj)
rescue LoadError
  pp(obj)
end
```

lib/irb/color_printer.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 require 'pp'
+require 'irb/color'
 
 module IRB
   class ColorPrinter < ::PP