test/dm: fix watchdog test

For successful execution of the watchdog test we need both the GPIO as well
as the SANDBOX watchdog.

Avoid a build failure for CONFIG_WDT_GPIO=n.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

test/dm/Makefile

@@ -106,6 +106,8 @@ obj-$(CONFIG_TIMER) += timer.o
 obj-$(CONFIG_DM_USB) += usb.o
 obj-$(CONFIG_DM_VIDEO) += video.o
 obj-$(CONFIG_VIRTIO_SANDBOX) += virtio.o
-obj-$(CONFIG_WDT) += wdt.o
+ifeq ($(CONFIG_WDT_GPIO)$(CONFIG_WDT_SANDBOX),yy)
+obj-y += wdt.o
+endif
 endif
 endif # !SPL