scripts/sortextable: silence script output

The exception table sorter outputs one line every time
it gets called, e.g. 'sort done marker at 66dc00', which
is slightly annoying when doing 'make -s' which is otherwise
completely silent. Since that output is not helpful to
most people building the kernel, turn it off by default.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: David Daney <david.daney@cavium.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>