http: add missing header bootlegged by boost < 1.72

httpserver.cpp:74:10: error: no template named 'deque' in namespace 'std'
    std::deque<std::unique_ptr<WorkItem>> queue;
    ~~~~~^

src/httpserver.cpp

@@ -15,6 +15,7 @@
 #include <sync.h>
 #include <ui_interface.h>
 
+#include <deque>
 #include <memory>
 #include <stdio.h>
 #include <stdlib.h>