powerpc/perf: Make symbol 'isa207_pmu_format_attr' static

The sparse tool complains as follows:

arch/powerpc/perf/isa207-common.c:24:18: warning:
 symbol 'isa207_pmu_format_attr' was not declared. Should it be static?

This symbol is not used outside of isa207-common.c, so this
commit marks it static.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Bixuan Cui <cuibixuan@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210409090119.59444-1-cuibixuan@huawei.com

arch/powerpc/perf/isa207-common.c

@@ -21,7 +21,7 @@ PMU_FORMAT_ATTR(thresh_stop,	"config:32-35");
 PMU_FORMAT_ATTR(thresh_start,	"config:36-39");
 PMU_FORMAT_ATTR(thresh_cmp,	"config:40-49");
 
-struct attribute *isa207_pmu_format_attr[] = {
+static struct attribute *isa207_pmu_format_attr[] = {
 	&format_attr_event.attr,
 	&format_attr_pmcxsel.attr,
 	&format_attr_mark.attr,