arm64: defconfig: enable taskstats configuration

Enable CONFIG_TASKSTATS which is required for CONFIG_TASK_XACCT (and
subsequently CONFIG_TASK_IO_ACCOUNTING). Previously, taskstats got
pulled in by KVM but that got changed in commit 63b3f96e
("kvm: Select SCHED_INFO instead of TASK_DELAY_ACCT").

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>