fdt: restore board_fdt_blob_setup() declaration

Commit 90c08fa0 (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>