powerpc/xmon: Make symbol 'spu_inst_dump' static

Fix sparse warning:

arch/powerpc/xmon/xmon.c:4216:1: warning:
 symbol 'spu_inst_dump' was not declared. Should it be static?

This symbol is not used outside of xmon.c, so make it static.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210409070151.163424-1-pulehui@huawei.com