tools/virtio: move module license stub to module.h

This fixes build for the vringh test:
[linux]$ make -C tools/virtio/
make: Entering directory `/home/mst/scm/linux/tools/virtio'
cc -g -O2 -Wall -I. -I ../../usr/include/ -Wno-pointer-sign
-fno-strict-overflow -fno-strict-aliasing -fno-common -MMD
-U_FORTIFY_SOURCE   -c -o vringh.o ../../drivers/vhost/vringh.c
../../drivers/vhost/vringh.c:1010:16: error: expected declaration
specifiers or ‘...’ before string constant

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>