sandbox: fix a compilation error

sandbox: fix a compilation error

With sandbox and sandbox64 configurations:

In file included from .../u-boot/include/test/test.h:156,
                 from .../u-boot/include/test/lib.h:9,
                 from .../u-boot/test/lib/test_crc8.c:8:
.../u-boot/arch/sandbox/include/asm/test.h: In function ‘sandbox_sdl_set_bpp’:
.../u-boot/arch/sandbox/include/asm/test.h:323:17: error: ‘ENOSYS’ undeclared (first use in this function)
  323 |         return -ENOSYS;
      |                 ^~~~~~

Per Tom Rini's suggestion:
move that function prototype over to arch/sandbox/include/asm/sdl.h
and make test/dm/video.c include <asm/sdl.h>

Cc: Simon Glass <sjg@chromium.org>
Suggested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>