binman: blob_dtb: Add fake_size argument to ObtainContents()

The method 'connect_contents_to_file()' calls ObtainsContents() with
'fake_size' argument. Without providing the argument in the blob_dtb
we are not able to call this method without error.

Signed-off-by: Lukas Funke <lukas.funke@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>