powerpc: Use generic posix_types.h

Change the powerpc architecture to use <asm-generic/posix_types.h>.

[ v2: fix the definition for __kernel_ssize_t ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/1328677745-20121-16-git-send-email-hpa@zytor.com
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>