ARM: binfmt_flat: unused variable 'persistent'

The flat_get_addr_from_rp() macro does not use the 'persistent' argument
on ARM, causing a harmless compiler warning. A cast to void removes
that warning.

Without this patch, building at91x40_defconfig results in:

fs/binfmt_flat.c: In function 'load_flat_file':
fs/binfmt_flat.c:746:17: warning: unused variable 'persistent' [-Wunused-variable]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Bryan Wu <bryan.wu@canonical.com>