powerpc/perf: Use uapi/unistd.h to fix build error

Use the 'unistd.h' from arch/powerpc/include/uapi to build the perf tool.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>