generic-ipi: powerpc/generic-ipi tree build failure

Today's linux-next build (powerpc allmodconfig) failed like this:

ERROR: ".save_stack_trace" [tests/backtracetest.ko] undefined!

But save_stack_trace is exported in arch/powerpc/kernel/stacktrace.c

I couldn't figure it out until I noticed these earlier warnings:

arch/powerpc/kernel/stacktrace.c:47: warning: data definition has no type or storage class
arch/powerpc/kernel/stacktrace.c:47: warning: type defaults to 'int' in declaration of 'EXPORT_SYMBOL_GPL'
arch/powerpc/kernel/stacktrace.c:47: warning: parameter names (without types) in function declaration

I applied the patch below.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: <linuxppc-dev@ozlabs.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>