http: suppress "flags may be used uninitialized in this function" error

http: suppress "flags may be used uninitialized in this function" error

Some GCC reports [1]:

    /home/runner/work/libevent/libevent/http.c: In function ‘evhttp_make_header’:
    /home/runner/work/libevent/libevent/http.c:503:14: error: ‘flags’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
      ev_uint16_t flags;
                  ^~~~~
    /home/runner/work/libevent/libevent/http.c: In function ‘evhttp_get_body’:
    /home/runner/work/libevent/libevent/http.c:2354:14: error: ‘flags’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
      ev_uint16_t flags;
                  ^~~~~
  [1]: https://github.com/libevent/libevent/runs/7263518338?check_suite_focus=true#logs