perf/powerpc: Fix build error

Fix compile errors like those below:

  CC      arch/powerpc/perf/power7-pmu.o
   /home/git/linux/arch/powerpc/perf/power7-pmu.c:397:2: error: initialization from incompatible pointer type [-Werror]

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Acked-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: linuxppc-dev@ozlabs.org
Link: http://lkml.kernel.org/r/20130205231938.GA24125@us.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>