Use typeset -g to avoid warnnestedvar warnings

Fixes github issue #271

src/strategies/default.zsh

@@ -11,5 +11,5 @@ _zsh_autosuggest_strategy_default() {
 
 	# Get the history items that match
 	# - (r) subscript flag makes the pattern match on values
-	suggestion="${history[(r)${(b)prefix}*]}"
+	typeset -g suggestion="${history[(r)${(b)prefix}*]}"
 }

src/strategies/match_prev_cmd.zsh

@@ -48,5 +48,5 @@ _zsh_autosuggest_strategy_match_prev_cmd() {
 	done
 
 	# Give back the matched history entry
-	suggestion="$history[$histkey]"
+	typeset -g suggestion="$history[$histkey]"
 }