configs/at91sam9g20dfc_defconfig: lock down kernel headers vesion

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

configs/at91sam9g20dfc_defconfig

@@ -2,6 +2,10 @@
 BR2_arm=y
 BR2_arm926t=y
 
+# Lock down headers to avoid breaking with new defaults
+BR2_KERNEL_HEADERS_VERSION=y
+BR2_DEFAULT_KERNEL_VERSION="3.1.6"
+
 # Host utilities
 #
 BR2_PACKAGE_HOST_SAM_BA=y