test-util-sockets: Add stub for monitor_set_cur()

Without this stub, the next commit fails to link.  I suspect the real
cause is 947e47448d "monitor: Use getter/setter functions for
cur_mon".

Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210318155519.1224118-9-armbru@redhat.com>

tests/unit/test-util-sockets.c

@@ -73,6 +73,7 @@ int monitor_get_fd(Monitor *mon, const char *fdname, Error **errp)
  * otherwise we get duplicate syms at link time.
  */
 Monitor *monitor_cur(void) { return cur_mon; }
+Monitor *monitor_set_cur(Coroutine *co, Monitor *mon) { abort(); }
 int monitor_vprintf(Monitor *mon, const char *fmt, va_list ap) { abort(); }
 
 #ifndef _WIN32