[gnome-sdk-images] Avoid which fail output

[Alexander Larsson <alexl src gnome org>]

commit 3b74c54923adbc34f8bf2957a22e4723426cebb7
Author: Alexander Larsson <alexl redhat com>
Date:   Tue Mar 24 13:37:03 2015 +0100

    Avoid which fail output

 build.sh |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/build.sh b/build.sh
index 7303c78..3cfb911 100755
--- a/build.sh
+++ b/build.sh
@@ -6,8 +6,8 @@ ROOT=$BUILD/root
 VAR=$BUILD/var
 SRC=`pwd`
 
-XDG_APP_HELPER=`which xdg-app-helper`
-LINUX_USER_CHROOT=`which linux-user-chroot`
+XDG_APP_HELPER=`which xdg-app-helper 2> /dev/null`
+LINUX_USER_CHROOT=`which linux-user-chroot 2> /dev/null`
 
 declare -x LC_ALL=en_US.utf8
 declare -x HOME=/self/packages