image-host: fix 'unknown error' error message

Fix error message like this:
Can't add verification data for node 'fdt-1' (<unknown error>)

We get unknown error because we decode error as fdt error
but actually it is system error.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/image-host.c b/tools/image-host.c
index 49ce743..5e01b85 100644
--- a/tools/image-host.c
+++ b/tools/image-host.c
@@ -1333,7 +1333,7 @@
 		if (ret) {
 			fprintf(stderr, "Can't add verification data for node '%s' (%s)\n",
 				fdt_get_name(fit, noffset, NULL),
-				fdt_strerror(ret));
+				strerror(-ret));
 			return ret;
 		}
 	}