selftests/powerpc: Remove duplicated include from tm-poison.c

Remove duplicated include.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210326064808.3262568-1-zhengyongjun3@huawei.com