ostree: fix tag for build constraint

Build tag 'ostree' is needed to match the constraint of
vendor/github.com/containers/storage/pkg/ostree/ostree.go
instead of that of
vendor/github.com/containers/storage/pkg/ostree/no_ostree.go

Signed-off-by: Silvano Cirujano Cuesta <silvano.cirujano-cuesta@siemens.com>

Closes: #1137
Approved by: giuseppe

ostree_tag.sh

@@ -1,4 +1,6 @@
 #!/bin/bash
-if ! pkg-config ostree-1 2> /dev/null ; then
+if pkg-config ostree-1 2> /dev/null ; then
+	echo ostree
+else
 	echo containers_image_ostree_stub
 fi