vhost: fix indentation in vhost_user.h

Change-Id: Ib77a219e02a5cde69293eb1f7002507cf4930ae3
Fixes: 90c0e244 ("vhost_user_nvme: add vhost user nvme target to SPDK")
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/407193


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>