test: Add udevice declaration to avoid build error

When including this file on a board other than sandbox (e.g by enabling
UNIT_TEST and CMD_SETEXPR) an results. Fix it by declaring struct udevice
first.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/include/dm/test.h b/include/dm/test.h
index 6ac6672..c5a9610 100644
--- a/include/dm/test.h
+++ b/include/dm/test.h
@@ -6,6 +6,8 @@
 #ifndef __DM_TEST_H
 #define __DM_TEST_H
 
+struct udevice;
+
 /**
  * struct dm_test_cdata - configuration data for test instance
  *