vhost: workaround the build dependency on mbuf header

vhost: workaround the build dependency on mbuf header

rte_mbuf struct is something more likely will be used only in vhost-user
net driver, while we have made vhost-user generic enough that it can
be used for implementing other drivers (such as vhost-user SCSI), they
have also include <rte_mbuf.h>. Otherwise, the build will be broken.

We could workaround it by using forward declaration, so that other
non-net drivers won't need include <rte_mbuf.h>.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>