sandbox: move a function prototype

Since host_get_dev_errr() is defined in drivers/block/sandbox.c,
the associated function prototype should be in a more appropriate
header file.

Fixes: commit 4101f6879256 ("dm: Drop the block_dev_desc_t typedef")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
diff --git a/include/sandboxblockdev.h b/include/sandboxblockdev.h
index 4ca9554..dc983f0 100644
--- a/include/sandboxblockdev.h
+++ b/include/sandboxblockdev.h
@@ -26,4 +26,6 @@
  */
 int host_dev_bind(int dev, char *filename, bool removable);
 
+int host_get_dev_err(int dev, struct blk_desc **blk_devp);
+
 #endif