Fix lint error.

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/util/strings.go

@@ -22,7 +22,7 @@ import "strings"
 // Comparison is case insensitive.
 func InStringSlice(ss []string, str string) bool {
 	for _, s := range ss {
-		if strings.ToLower(s) == strings.ToLower(str) {
+		if strings.EqualFold(s, str) {
 			return true
 		}
 	}
@@ -34,7 +34,7 @@ func InStringSlice(ss []string, str string) bool {
 func SubtractStringSlice(ss []string, str string) []string {
 	var res []string
 	for _, s := range ss {
-		if strings.ToLower(s) == strings.ToLower(str) {
+		if strings.EqualFold(s, str) {
 			continue
 		}
 		res = append(res, s)