[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: NRobert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

include/asm-powerpc/Kbuild

 include include/asm-generic/Kbuild.asm
 
+header-y += a.out.h
 header-y += auxvec.h
 header-y += ioctls.h
 header-y += mman.h
@@ -23,7 +24,6 @@ header-y += sigcontext.h
 header-y += statfs.h
 header-y += ps3fb.h
 
-unifdef-y += a.out.h
 unifdef-y += asm-compat.h
 unifdef-y += bootx.h
 unifdef-y += byteorder.h