zsh/completion: put compdef on first line

Some zsh setups (including mine) do not load the
completion if `#compdef` is not on the first line.

So we move the `# shellcheck` comment to the
second line to avoid this issue.

Author: bryango

misc/zsh/completion.zsh

@@ -1,5 +1,5 @@
-# shellcheck disable=all
 #compdef nix
+# shellcheck disable=all
 
 function _nix() {
   local ifs_bk="$IFS"