[POWERPC] Move a.out.h to header-y since it doesn't check __KERNEL__

Since a.out.h doesn't check the value of __KERNEL__, there's no point
in unifdef'ing it.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>