fs: fs-loader: Drop dm.h header file

This header file should not be included in other header files. Remove it
and use a forward declaration instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/fs_loader.h b/include/fs_loader.h
index b728c06..1b3c580 100644
--- a/include/fs_loader.h
+++ b/include/fs_loader.h
@@ -6,7 +6,7 @@
 #ifndef _FS_LOADER_H_
 #define _FS_LOADER_H_
 
-#include <dm.h>
+struct udevice;
 
 /**
  * struct phandle_part - A place for storing phandle of node and its partition