Add missing vector include

```
In file included from option_strings.cc:43:
./../../torrent/connection_manager.h:246:8: error: ‘vector’ in namespace ‘std’ does not name a template type
  246 |   std::vector<TrackerUdp*> m_tracker_udp_list;
      |        ^~~~~~
./../../torrent/connection_manager.h:51:1: note: ‘std::vector’ is defined in header ‘<vector>’; did you forget to ‘#include <vector>’?
   50 | #include <torrent/common.h>
  +++ |+#include <vector>
```

libtorrent/src/torrent/connection_manager.h

@@ -49,6 +49,10 @@
 #include lt_tr1_functional
 #include <torrent/common.h>
 
+#ifdef USE_UDNS
+#include <vector>
+#endif
+
 namespace torrent {
 
 #ifdef USE_UDNS