[editor] Ensure unbound_keys is defined as an array

Somewhere around zsh 5.1, there was a change which turned typeset (and
most likely local) into a keyword. In older versions of zsh, using
`local x=()` will cause the () to be treated as a glob qualifier.

Fixes #1373

modules/editor/init.zsh

@@ -289,7 +289,8 @@ fi
 # it will fall back and do nothing.
 function _prezto-zle-noop {  ; }
 zle -N _prezto-zle-noop
-local unbound_keys=(
+local -a unbound_keys
+unbound_keys=(
   "${key_info[F1]}"
   "${key_info[F2]}"
   "${key_info[F3]}"