Fix build with NDEBUG defined

Fix build with NDEBUG defined

The usage of the gcc option -Wunused-parameter interferes badly with
the assert() macros.  In case -DNDEBUG is specified build fails with:

  cc1: warnings being treated as errors
  print.c: In function 'ipset_print_family':
  print.c:92: error: unused parameter 'opt'
  print.c: In function 'ipset_print_port':
  print.c:413: error: unused parameter 'opt'
  print.c: In function 'ipset_print_proto':

Fix it by taking into accout NDEBUG in the function arguments.

Bug reported by Holger Eitzenberger.