powerpc/perf: Fix symbol undeclared warning

Build kernel with `C=2`:
arch/powerpc/perf/isa207-common.c:24:18: warning: symbol
'isa207_pmu_format_attr' was not declared. Should it be static?
arch/powerpc/perf/power9-pmu.c:101:5: warning: symbol 'p9_dd21_bl_ev'
was not declared. Should it be static?
arch/powerpc/perf/power9-pmu.c:115:5: warning: symbol 'p9_dd22_bl_ev'
was not declared. Should it be static?

Those symbols are used only in the files that define them so we declare
them as static to fix the warnings.

Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
Reviewed-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200923071453.2540-1-wangwensheng4@huawei.com