stacktrace: fix build failure on sparc64

Today's linux-next build (spac64 allmodconfig) failed like this:

arch/sparc64/kernel/stacktrace.c:50: warning: type defaults to `int' in declaration of `EXPORT_SYMBOL_GPL'
arch/sparc64/kernel/stacktrace.c:50: warning: parameter names (without types) in function declaration
arch/sparc64/kernel/stacktrace.c:50: warning: data definition has no type or storage class

Signed-off-by: Stephen Rothwell <sf@canb.auug.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>