drm/amd/autoconf: fix check for hwmon_device_register_with_groups()

<linux/stddef.h> is needed for NULL pointer definition
this is for rhel 7.4/7.5/7.6

Signed-off-by: Flora Cui <flora.cui@amd.com>
Reviewed-by: Jack Gui <Jack.Gui@amd.com>

drivers/gpu/drm/amd/dkms/m4/hwmon-device-register-with-groups.m4

@@ -7,6 +7,7 @@ dnl # to the hwmon device directly and no longer with its parent device.
 AC_DEFUN([AC_AMDGPU_4ARGS_HWMON_DEVICE_REGISTER_WITH_GROUPS],
 	[AC_MSG_CHECKING([whether hwmon_device_register_with_groups() wants 4 args])
 	AC_KERNEL_TRY_COMPILE_SYMBOL([
+		#include <linux/stddef.h>
 		#include <linux/hwmon.h>
 	], [
 		hwmon_device_register_with_groups(NULL, NULL, NULL, NULL);