bswap: fix compiler warning

bswap: fix compiler warning

The bswap functions use memcpy but the bswap.h header itself does not seem to
include it in some configuration such as cross compiling for powerpc64
on x86_64 machine.

The patch explicitly includes string.h.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>