hwmon: (vexpress) Fix build error seen if CONFIG_OF_DEVICE is not set

Fix:
vexpress.c: In function ‘vexpress_hwmon_name_show’:
vexpress.c:34:2: error: implicit declaration of function
‘of_get_property’ [-Werror=implicit-function-declaration]
vexpress.c:34:27: warning: initialization makes pointer from integer
without a cast [enabled by default]
vexpress.c: In function ‘vexpress_hwmon_label_show’:
vexpress.c:43:22: warning: initialization makes pointer from integer
without a cast [enabled by default]

Seen if CONFIG_OF_DEVICE is not defined. of_get_property is declared in
of.h which is only included by of_device.h if CONFIG_OF_DEVICE is defined.
of.h needs to be included directly.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>