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