[gnome-sdk-images] Add a target to validate the json

[Matthias Clasen <matthiasc src gnome org>]

commit b6719d9241b22382f96db1390ef3f1db55308c6d
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Apr 16 00:17:17 2016 -0400

    Add a target to validate the json
    
    Can't hurt.

 Makefile |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/Makefile b/Makefile
index e99c347..231a00a 100644
--- a/Makefile
+++ b/Makefile
@@ -5,3 +5,6 @@ all: repo org.gnome.Sdk.json
 
 repo:
        ostree  init --mode=archive-z2 --repo=repo
+
+check:
+       json-glib-validate org.gnome.Sdk.json