selftests/powerpc: Fix duplicate included pthread.h

Clean up the following includecheck warning:

./tools/testing/selftests/powerpc/tm/tm-vmx-unavail.c: pthread.h is
included more than once.

No functional change.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1620903820-68213-1-git-send-email-jiapeng.chong@linux.alibaba.com