[gnome-sdk-images] Try to fix gcab build

[Michael Catanzaro <mcatanzaro src gnome org>]

commit f92032cd7d18de0cba386691fa25d413b7b92eb8
Author: Michael Catanzaro <mcatanzaro igalia com>
Date:   Thu May 31 09:27:52 2018 -0500

    Try to fix gcab build

 org.gnome.Sdk.json.in | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/org.gnome.Sdk.json.in b/org.gnome.Sdk.json.in
index fd12d13..97a64f5 100644
--- a/org.gnome.Sdk.json.in
+++ b/org.gnome.Sdk.json.in
@@ -695,6 +695,7 @@
         {
             "name": "gcab",
             "buildsystem": "meson",
+            "config-opts": ["-Ddocs=false"],
             "ensure-writable": [
                 "/share/locale/*/LC_MESSAGES/gcab.mo",
                 "/share/runtime/locale/*/share/*/LC_MESSAGES/gcab.mo"