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>