selftests/timers: Fix spelling mistake "clocksourc" -> "clocksource"

There is a spelling mistake in a comment. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>