Fix libfdt warnings on Darwin

Building with libfdt results in the following warnings on Mac OS X:

  CC    ppc-softmmu/device_tree.o
In file included from /Users/andreas/QEMU/latest64/include/libfdt.h:54,
                 from /Users/andreas/QEMU/qemu/device_tree.c:26:
/Users/andreas/QEMU/qemu/libfdt_env.h:25:20: warning: endian.h: No such file or directory
/Users/andreas/QEMU/qemu/libfdt_env.h:26:22: warning: byteswap.h: No such file or directory
/Users/andreas/QEMU/qemu/libfdt_env.h:28:5: warning: "__BYTE_ORDER" is not defined
/Users/andreas/QEMU/qemu/libfdt_env.h:28:21: warning: "__BIG_ENDIAN" is not defined

Since QEMU's copy of libfdt_env.h only uses bswap_32() and bswap_64(),
let QEMU's bswap.h take care of the headers and use its endianness define.

Cc: Hollis Blanchard <hollis@penguinppc.org>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Acked-by: Alexander Graf <agraf@suse.de>