sandbox: fix build error due to missing struct udevice definition

Without this patch, compiling may potentially fail.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/u-boot-sandbox.h b/arch/sandbox/include/asm/u-boot-sandbox.h
index b2b8e36..798d003 100644
--- a/arch/sandbox/include/asm/u-boot-sandbox.h
+++ b/arch/sandbox/include/asm/u-boot-sandbox.h
@@ -26,6 +26,8 @@
 /* drivers/video/sandbox_sdl.c */
 int sandbox_lcd_sdl_early_init(void);
 
+struct udevice;
+
 /**
  * pci_map_physmem() - map a PCI device into memory
  *