Silence unused-but-set-variable warnings

Silence unused-but-set-variable warnings

When ipset is compiled in non-debug mode, in some environments warnings
treated as errors emitted:

  session.c: In function 'build_msg':
  session.c:1985:28: warning: variable 'type' set but not used
[-Wunused-but-set-variable]
     const struct ipset_type *type;
                            ^
  session.c:2030:28: warning: variable 'type' set but not used
[-Wunused-but-set-variable]
     const struct ipset_type *type;
                            ^
Fix it by hiding the unused variable definitions/settings in non-debug mode.

Reported by Serhey Popovych.

Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>