ppc: avoid typedef redefinitions

These cause compilation failures on CentOS 6 or other operating
systems with older GCCs.

Cc: David Gibson <dgibson@redhat.com>
Cc: qemu-ppc@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1488558530-21016-3-git-send-email-pbonzini@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>