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: NSimon Glass <sjg@chromium.org>
Reported-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

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
  *