selftests: timers: clocksource-switch: sort includes

It is easier to check if you need to add an include if the existing ones
are sorted.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>