ARM: arch_timer: include linux/errno.h

Patch 8a4da6e3 "arm: arch_timer: move core to drivers/clocksource"
moved a lot of code out of arch_timer.c, but ended up deleting
too much, which broke some configurations.

Obviously, include linux/errno.h is required to return error
values.

Without this patch, building allmodconfig results in:

arch/arm/kernel/arch_timer.c: In function 'arch_timer_sched_clock_init':
arch/arm/kernel/arch_timer.c:55:11: error: 'ENXIO' undeclared (first use in this function)
arch/arm/kernel/arch_timer.c:55:11: note: each undeclared identifier is reported only once for each function it appears in
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>

arch/arm/kernel/arch_timer.c

@@ -10,6 +10,7 @@
  */
 #include <linux/init.h>
 #include <linux/types.h>
+#include <linux/errno.h>
 
 #include <asm/delay.h>
 #include <asm/sched_clock.h>