migration: drop an unused include

In the vmstate.h file, we just need a struct name. Use a forward
declaration instead of an include, then adjust the one affected .c file
to include the file that is no longer implicit from the header.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
2 files changed