configs: Add IPV6 config to sandbox_flattree_defconfig

Allow to use IPV6 tests in sandbox

Signed-off-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index 01234dc..7237efe 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -216,3 +216,4 @@
 CONFIG_UNIT_TEST=y
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y
+CONFIG_IPV6=y