virtio: remove inclusion of mbuf header

virtio: remove inclusion of mbuf header

The virtqueue header files include the mbuf header file, but they do not
need to do so as they only use pointers to the struct rte_mbuf type, and
do not use any of the mbuf internals, nor any of the mbuf functions or
macros. Therefore the inclusion is unnecessary, and can be replaced by a
forward declaration of the mbuf type.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>