Set libstrophe log verbosity

Set this to 0.
We might want to have this configurable later.

For now we fix the valgrind report:

```
Conditional jump or move depends on uninitialised value
xmpp_debug_verbose()
```

Which will be fixed in libstrophe > 0.11.0 by commit
28f3ce19b8

src/xmpp/connection.c

@@ -170,6 +170,7 @@ connection_connect(const char* const jid, const char* const passwd, const char*
         log_warning("Failed to get libstrophe ctx during connect");
         return JABBER_DISCONNECTED;
     }
+    xmpp_ctx_set_verbosity(conn.xmpp_ctx, 0);
     conn.xmpp_conn = xmpp_conn_new(conn.xmpp_ctx);
     if (conn.xmpp_conn == NULL) {
         log_warning("Failed to get libstrophe conn during connect");