Add missing require

This is used for `hash-table-keys`.

Fix https://github.com/jwiegley/use-package/issues/644.

lisp/use-package/use-package-core.el

@@ -42,6 +42,7 @@
 (require 'bytecomp)
 (require 'cl-lib)
 (require 'tabulated-list)
+(require 'subr-x)
 
 (eval-when-compile
   (require 'cl)