add build tags to disable kms providers

kms/gcp_disabled.go

@@ -0,0 +1,17 @@
+// +build nogcpkms
+
+package kms
+
+import (
+	"errors"
+
+	"github.com/drakkan/sftpgo/version"
+)
+
+func init() {
+	version.AddFeature("-gcpkms")
+}
+
+func newGCPSecret(base baseSecret, url, masterKey string) SecretProvider {
+	return newDisabledSecret(errors.New("GCP KMS disabled at build time"))
+}