[gnome-sdk-images] Makefile: Add rules to generate .json from .in

[Alexander Larsson <alexl src gnome org>]

commit 2b077acfc33306b055be97b4360718c090bdfb13
Author: Alexander Larsson <alexl redhat com>
Date:   Tue May 8 09:47:31 2018 +0200

    Makefile: Add rules to generate .json from .in

 Makefile | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/Makefile b/Makefile
index 98995fe..3d715e3 100644
--- a/Makefile
+++ b/Makefile
@@ -30,6 +30,9 @@ all: ${REPO} $(patsubst %,%.in,$(SUBST_FILES))
                         --subject="build of org.gnome.Sdk, `date`" \
                         ${EXPORT_ARGS} sdk org.gnome.Sdk.json
 
+$(SUBST_FILES): $(patsubst %,%.in,$(SUBST_FILES))
+       $(call subst-metadata)
+
 ${REPO}:
        ostree  init --mode=archive-z2 --repo=${REPO}