virtfs-proxy-helper: add missing long option terminator

The getopt_long(3) long options array must have a zeroed terminator.

This patch solves a segmentation fault when an unknown command-line
option is encountered:

  $ fsdev/virtfs-proxy-helper --help
  Segmentation fault (core dumped)
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

fsdev/virtfs-proxy-helper.c

@@ -49,6 +49,7 @@ static struct option helper_opts[] = {
     {"socket", required_argument, NULL, 's'},
     {"uid", required_argument, NULL, 'u'},
     {"gid", required_argument, NULL, 'g'},
+    {},
 };
 
 static bool is_daemon;