powerpc/papr_scm: Fix warnings about undeclared variable

Build the kernel with 'make C=2':
arch/powerpc/platforms/pseries/papr_scm.c:825:1: warning: symbol
'dev_attr_perf_stats' was not declared. Should it be static?
Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
Reviewed-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200918085951.44983-1-wangwensheng4@huawei.com

arch/powerpc/platforms/pseries/papr_scm.c

@@ -823,7 +823,7 @@ static ssize_t perf_stats_show(struct device *dev,
 	kfree(stats);
 	return rc ? rc : (ssize_t)seq_buf_used(&s);
 }
-DEVICE_ATTR_ADMIN_RO(perf_stats);
+static DEVICE_ATTR_ADMIN_RO(perf_stats);
 
 static ssize_t flags_show(struct device *dev,
 			  struct device_attribute *attr, char *buf)