powerpc/perf_counter: Remove duplicated #include

Remove duplicated #include('s) in
  arch/powerpc/kernel/mpc7450-pmu.c
  arch/powerpc/kernel/ppc970-pmu.c

Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>