dnsdist: Prevent a false-positive warning from the compiler

dnsdist: Prevent a false-positive warning from the compiler

```
In file included from xsk.hh:50,
                 from xsk.cc:23:
In function ‘xdp_desc* xsk_ring_prod__tx_desc(xsk_ring_prod*, __u32)’,
    inlined from ‘void XskSocket::send(std::vector<std::unique_ptr<XskPacket>, std::allocator<std::unique_ptr<XskPacket> > >&)’ at xsk.cc:176:28:
/usr/include/xdp/xsk.h:76:27: warning: ‘idx’ may be used uninitialized [-Wmaybe-uninitialized]
   76 |         return &descs[idx & tx->mask];
      |                       ~~~~^~~~~~~~~~
```