srd_decoder_unload_all(): Fix a -Wcast-function-type compiler warning.

  decoder.c: In function ‘srd_decoder_unload_all’:
  decoder.c:1080:27: warning: cast between incompatible function types from ‘int (*)(struct srd_decoder *)’ to ‘void (*)(void *, void *)’ [-Wcast-function-type]
    g_slist_foreach(pd_list, (GFunc)srd_decoder_unload, NULL);
                           ^