[ARM] stacktrace fix

ab1b6f03

 said

 - remove the unused task argument to save_stack_trace, it's always current

then broke arm:

arch/arm/kernel/stacktrace.c:56: error: conflicting types for 'save_stack_trace'
include/linux/stacktrace.h:11: error: previous declaration of 'save_stack_trace' was here
arch/arm/kernel/stacktrace.c:56: error: conflicting types for 'save_stack_trace'
include/linux/stacktrace.h:11: error: previous declaration of 'save_stack_trace' was here

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>