add build tags to disable kms providers

kms/aws.go

@@ -1,13 +1,22 @@
+// +build !noawskms
+
 package kms
 
-const (
-	awsProviderName = "AWS"
+import (
+	// we import awskms here to be able to disable AWS KMS support using a build tag
+	_ "gocloud.dev/secrets/awskms"
+
+	"github.com/drakkan/sftpgo/version"
 )
 
 type awsSecret struct {
 	baseGCloudSecret
 }
 
+func init() {
+	version.AddFeature("+awskms")
+}
+
 func newAWSSecret(base baseSecret, url, masterKey string) SecretProvider {
 	return &awsSecret{
 		baseGCloudSecret{