buildroot: sync submodule to use an updated skiroot

Signed-off-by: Jeremy Kerr <jeremy.kerr@au.ibm.com>
diff --git a/buildroot b/buildroot
index 6b2959c..1dd9b67 160000
--- a/buildroot
+++ b/buildroot
@@ -1 +1 @@
-Subproject commit 6b2959c880d08766a1ace9e14f6bb6272a7b4688
+Subproject commit 1dd9b679cb299878c37e2839e5f1b0695ce76484