vmxnet3: Eliminate __packed redefined warning

This eliminates a warning about __packed being redefined as exposed by the
vmxnet3 code. __packed is not used anywhere in the vmxnet3 code.

  CC    hw/net/vmxnet3.o
In file included from hw/net/vmxnet3.c:29:
hw/net/vmxnet3.h:37:1: warning: "__packed" redefined
In file included from /usr/include/stdlib.h:38,
                 from /buildbot-qemu/default_openbsd_current/build/include/qemu-common.h:26,
                 from /buildbot-qemu/default_openbsd_current/build/include/hw/hw.h:5,
                 from hw/net/vmxnet3.c:18:
/usr/include/sys/cdefs.h:209:1: warning: this is the location of the previous definition

Signed-off-by: Brad Smith <brad@comstyle.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
