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>

tools/virtio/linux/module.h

 #include <linux/export.h>
+
+#define MODULE_LICENSE(__MODULE_LICENSE_value) \
+	static __attribute__((unused)) const char *__MODULE_LICENSE_name = \
+		__MODULE_LICENSE_value
+

tools/virtio/linux/virtio.h

@@ -45,9 +45,6 @@ struct virtqueue {
 	void *priv;
 };
 
-#define MODULE_LICENSE(__MODULE_LICENSE_value) \
-	const char *__MODULE_LICENSE_name = __MODULE_LICENSE_value
-
 /* Interfaces exported by virtio_ring. */
 int virtqueue_add_sgs(struct virtqueue *vq,
 		      struct scatterlist *sgs[],