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>

arch/powerpc/kernel/mpc7450-pmu.c

@@ -10,7 +10,6 @@
  */
 #include <linux/string.h>
 #include <linux/perf_counter.h>
-#include <linux/string.h>
 #include <asm/reg.h>
 #include <asm/cputable.h>
 

arch/powerpc/kernel/ppc970-pmu.c

@@ -10,7 +10,6 @@
  */
 #include <linux/string.h>
 #include <linux/perf_counter.h>
-#include <linux/string.h>
 #include <asm/reg.h>
 #include <asm/cputable.h>