component/postfix: build without NIS.

To avoid possible build failure like the following :

  ranlib ../../lib/libmaster.a
  /.../bin/ld: ../../lib/libutil.a(dict_nis.o): in function `dict_nis_lookup':
  /.../postfix-3.7.3/src/util/dict_nis.c:173: undefined reference to `yp_match'
  /.../bin/ld: /.../postfix-3.7.3/src/util/dict_nis.c:187: undefined reference to `yp_match'
  /.../bin/ld: ../../lib/libutil.a(dict_nis.o): in function `dict_nis_init':
  /.../postfix-3.7.3/src/util/dict_nis.c:81: undefined reference to `yp_get_default_domain'