BUILD: sink: address build warning on 32-bit architectures

BUILD: sink: address build warning on 32-bit architectures

The recently added ring section post-processing added this bening
warning on 32-bit archs:

  src/sink.c: In function 'cfg_post_parse_ring':
  src/sink.c:994:15: warning: format '%lu' expects argument of type 'long unsigned int', but argument 4 has type 'size_t {aka unsigned int}' [-Wformat=]
      ha_warning("ring '%s' event max length '%u' exceeds size, forced to size '%lu'.\n",
               ^
Let's just cast b_size() to unsigned long here.