clocksource/drivers/pxa: Fix include files for compilation

We get 1 warning about global functions without a declaration in the
 clocksource/drivers/pxa driver when building with W=1:

drivers/clocksource/pxa_timer.c:221:13: warning: no previous prototype for 'pxa_timer_nodt_init' [-Wmissing-prototypes]
 void __init pxa_timer_nodt_init(int irq, void __iomem *base,

In fact, this function is declared in pxa.h, so this patch
add missing header dependencies.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Cc: daniel.lezcano@linaro.org
Cc: xie.baoyou@zte.com.cn
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1471965569-4104-1-git-send-email-baoyou.xie@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

drivers/clocksource/pxa_timer.c

@@ -21,6 +21,8 @@
 #include <linux/of_irq.h>
 #include <linux/sched_clock.h>
 
+#include <clocksource/pxa.h>
+
 #include <asm/div64.h>
 
 #define OSMR0		0x00	/* OS Timer 0 Match Register */