[gnome-sdk-images] Use the buildsystem property

[Mathieu Bridon <mbridon src gnome org>]

commit c51a43560ed7087d259a40e7253e372aba8ecdb8
Author: Mathieu Bridon <bochecha daitauha fr>
Date:   Mon Mar 20 10:12:30 2017 +0100

    Use the buildsystem property
    
    This is the new, preferred way to specify that a module uses cmake.

 org.gnome.Sdk.json.in |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/org.gnome.Sdk.json.in b/org.gnome.Sdk.json.in
index 6e97912..40c6cd2 100644
--- a/org.gnome.Sdk.json.in
+++ b/org.gnome.Sdk.json.in
@@ -727,7 +727,7 @@
         {
             "name": "webkitgtk4",
             "cleanup-platform": [ "/libexec/webkit2gtk-4.0/MiniBrowser" ],
-            "cmake": true,
+            "buildsystem": "cmake",
             "build-options" : {
                 "cflags": "-g1",
                 "cxxflags": "-g1",