powerpc: Include the appropriate endianness header

This patch will have powerpc include the appropriate generic endianness
header depending on what the compiler reports.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>