powerpc/pseries: Make symbol '__pcpu_scope_hcall_stats' static

The sparse tool complains as follows:

arch/powerpc/platforms/pseries/hvCall_inst.c:29:1: warning:
 symbol '__pcpu_scope_hcall_stats' was not declared. Should it be static?

This symbol is not used outside of hvCall_inst.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/20210409090109.59347-1-cuibixuan@huawei.com