Detech -Wno-unused-functions for GCC too

Detech -Wno-unused-functions for GCC too

Otherwise build fails [1]:

    /home/runner/work/libevent/libevent/dist/libevent-2.2.0-alpha-dev/minheap-internal.h:64:6: error: ‘min_heap_ctor_’ defined but not used [-Werror=unused-function]
     void min_heap_ctor_(min_heap_t* s) { s->p = 0; s->n = 0; s->a = 0; }
          ^~~~~~~~~~~~~~
    In file included from /home/runner/work/libevent/libevent/dist/libevent-2.2.0-alpha-dev/buffer.c:91:0:
    /home/runner/work/libevent/libevent/dist/libevent-2.2.0-alpha-dev/bufferevent-internal.h:414:1: error: ‘bufferevent_trigger_nolock_’ defined but not used [-Werror=unused-function]
     bufferevent_trigger_nolock_(struct bufferevent *bufev, short iotype, int options)
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~
    In file included from /home/runner/work/libevent/libevent/dist/libevent-2.2.0-alpha-dev/buffer.c:89:0:
    /home/runner/work/libevent/libevent/dist/libevent-2.2.0-alpha-dev/evthread-internal.h:140:1: error: ‘EVLOCK_TRY_LOCK_’ defined but not used [-Werror=unused-function]
     EVLOCK_TRY_LOCK_(void *lock)

  [1]: https://github.com/libevent/libevent/runs/7263518180?check_suite_focus=true