omap2plus: voltage: Trivial linking fix for 'EINVAL' undeclared

CC      arch/arm/mach-omap2/omap_hwmod_common_data.o
In file included from arch/arm/plat-omap/include/plat/omap_hwmod.h:38,
                 from arch/arm/mach-omap2/omap_hwmod_common_data.c:20:
arch/arm/plat-omap/include/plat/voltage.h: In function 'omap_voltage_late_init':
arch/arm/plat-omap/include/plat/voltage.h:145: error: 'EINVAL' undeclared (first use in this function)
arch/arm/plat-omap/include/plat/voltage.h:145: error: (Each undeclared identifier is reported only once
arch/arm/plat-omap/include/plat/voltage.h:145: error: for each function it appears in.)
make[1]: *** [arch/arm/mach-omap2/omap_hwmod_common_data.o] Error 1
make: *** [arch/arm/mach-omap2] Error 2

The error is reported when omap2plus_defconfig built with CONFIG_PM disabled
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Thara Gopinath <thara@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>

arch/arm/plat-omap/include/plat/voltage.h

@@ -14,6 +14,8 @@
 #ifndef __ARCH_ARM_MACH_OMAP2_VOLTAGE_H
 #define __ARCH_ARM_MACH_OMAP2_VOLTAGE_H
 
+#include <linux/err.h>
+
 #define VOLTSCALE_VPFORCEUPDATE		1
 #define VOLTSCALE_VCBYPASS		2