Add missing require statement for seq

This probably never made a difference in practice because seq always
gets loaded by something or other anyway. But now it's actually
correct.

ido-completing-read+.el

@@ -84,6 +84,7 @@ Note that when you update ido-completing-read+, this variable may
 not be updated until you restart Emacs.")
 
 (require 'ido)
+(require 'seq)
 (require 'minibuf-eldef)
 (require 'cl-lib)
 (require 'cus-edit)