net/virtio: fix build

Add extern to variable declaration to avoid some compiler treating it
as variable definition.

build error log:

lib/librte_pmd_virtio.a(vhost_kernel.o):(.rodata+0x110):
multiple definition of `vhost_msg_strings'
lib/librte_pmd_virtio.a(vhost_user.o):(.data.rel.ro.local+0x0):
first defined here
lib/librte_pmd_virtio.a(virtio_user_dev.o):(.rodata+0xe8):
multiple definition of `vhost_msg_strings'
lib/librte_pmd_virtio.a(vhost_user.o):(.data.rel.ro.local+0x0):
first defined here

Fixes: 33d24d65fe2b ("net/virtio-user: abstract backend operations")
Cc: stable@dpdk.org
Signed-off-by: Jerin Jacob <jerinj@marvell.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>