byteorder: make swab.h include asm/swab.h like a regular header

Add swab.h to kbuild.asm and remove the individual entries from
each arch, mark as unifdef as some arches have some kernel-only
bits inside.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/xtensa/include/asm/Kbuild

@@ -1,3 +1 @@
 include include/asm-generic/Kbuild.asm
-
-unifdef-y += swab.h

arch/xtensa/include/asm/byteorder.h

@@ -1,8 +1,6 @@
 #ifndef _XTENSA_BYTEORDER_H
 #define _XTENSA_BYTEORDER_H
 
-#include <asm/swab.h>
-
 #ifdef __XTENSA_EL__
 #include <linux/byteorder/little_endian.h>
 #elif defined(__XTENSA_EB__)