fdt: restore board_fdt_blob_setup() declaration

Commit 90c08fa038451d (fdt: Add device tree memory bindings) removed the
prototype declaration of board_fdt_blob_setup(), most likely by mistake.
This didn't break the build because the only file calling this function
(lib/fdtdec.c) provides a local weak definition. Restore the
declaration.

Cc: Michael Pratt <mpratt@chromium.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/fdtdec.h b/include/fdtdec.h
index c26df50..b15da00 100644
--- a/include/fdtdec.h
+++ b/include/fdtdec.h
@@ -956,6 +956,7 @@
  * Called when CONFIG_OF_BOARD is defined, or if CONFIG_OF_SEPARATE is defined
  * and the board implements it.
  */
+void *board_fdt_blob_setup(void);
 
 /*
  * Decode the size of memory