[gnome-sdk-images] Pick up arch using flatpak, not xdg-app

[Alexander Larsson <alexl src gnome org>]

commit 9869e111e2dac7080de8112f9fd78a5cb08b5feb
Author: Alexander Larsson <alexl redhat com>
Date:   Mon May 23 13:11:16 2016 +0200

    Pick up arch using flatpak, not xdg-app

 Makefile |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/Makefile b/Makefile
index 9301292..088967e 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 # Override the arch with `make ARCH=i386`
-ARCH   ?= $(shell xdg-app --default-arch)
+ARCH   ?= $(shell flatpak --default-arch)
 REPO   ?= repo
 
 # SDK Versions setup here