clocksource: vf_pit_timer: Use linux/sched_clock.h

Commit 38ff87f7 (sched_clock: Make ARM's sched_clock generic for all
architectures) changed the header to <linux/sched_clock.h>, so adapt
it in order to fix the following build error:

drivers/clocksource/vf_pit_timer.c:15:29: fatal error: asm/sched_clock.h: No such file or directory

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Cc: shawn.guo@linaro.org
Cc: sboyd@codeaurora.org
Cc: john.stultz@linaro.org
Link: http://lkml.kernel.org/r/1372116008-2323-1-git-send-email-festevam@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>