server : add --flash-attn usage output (#3152)

This commit adds the `--flash-attn` option to the usage output of the
server example.

The motivation for this change is that while it is possible to set this
option it is not printed in the usage output.

examples/server/server.cpp

@@ -139,6 +139,7 @@ void whisper_print_usage(int /*argc*/, char ** argv, const whisper_params & para
     fprintf(stderr, "  -nth N,    --no-speech-thold N [%-7.2f] no speech threshold\n",   params.no_speech_thold);
     fprintf(stderr, "  -nc,       --no-context        [%-7s] do not use previous audio context\n", params.no_context ? "true" : "false");
     fprintf(stderr, "  -ng,       --no-gpu            [%-7s] do not use gpu\n", params.use_gpu ? "false" : "true");
+    fprintf(stderr, "  -fa,       --flash-attn        [%-7s] flash attention\n", params.flash_attn ? "true" : "false");
     fprintf(stderr, "\n");
 }