xtensa: Use generic posix_types.h

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

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/1328677745-20121-21-git-send-email-hpa@zytor.com
Cc: Chris Zankel <chris@zankel.net>