libvhost-user: make it a meson subproject

By making libvhost-user a subproject, check it builds
standalone (without the global QEMU cflags etc).

Note that the library still relies on QEMU include/qemu/atomic.h and
linux_headers/.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20201125100640.366523-6-marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/tests/vhost-user-bridge.c b/tests/vhost-user-bridge.c
index bd43607..2481592 100644
--- a/tests/vhost-user-bridge.c
+++ b/tests/vhost-user-bridge.c
@@ -34,7 +34,7 @@
 #include "qemu/ctype.h"
 #include "qemu/iov.h"
 #include "standard-headers/linux/virtio_net.h"
-#include "contrib/libvhost-user/libvhost-user.h"
+#include "libvhost-user.h"
 
 #define VHOST_USER_BRIDGE_DEBUG 1