Add missing license header on one file
Add missing license header on one file

 variables-parameters-constants.lsp was missing a license header
 and (at the time of this commit) had only ever been touched by
 the one first commit in 2013, so I added the samed header as in
 the other files, with the same copyright year (2013).

farthVader91/lisp-koans,google/lisp-koans,farthVader91/lisp-koans,google/lisp-koans