[gnome-builder/gnome-builder-3-22] flatpak: Fix build on flatpak-builder 0.9

[Alexander Larsson <alexl src gnome org>]

commit c12c8630cc30915986a1bbc96220f573261f0e33
Author: Alexander Larsson <alexl redhat com>
Date:   Thu Mar 9 08:44:40 2017 +0100

    flatpak: Fix build on flatpak-builder 0.9

 org.gnome.Builder.json |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/org.gnome.Builder.json b/org.gnome.Builder.json
index bd534c6..6b4bf77 100644
--- a/org.gnome.Builder.json
+++ b/org.gnome.Builder.json
@@ -161,6 +161,7 @@
         {
             "name": "python3-lxml",
             "no-autogen": true,
+            "ensure-writable": ["/lib/python*/site-packages/easy-install.pth"],
             "build-options": {
                 "cflags": "-I/usr/include",
                 "env": {