lib: fix handling of missing const type qualifiers.

[heitbaum]

For ISO C23, the functions strstr and strpbrk that return pointers into their input arrays now have definitions as macros that return a pointer to a const-qualified type when the in put argument is a pointer to a const-qualified type.

Update to const type for variabkes, as returned string is only used in comparisons which const can be used.

fixes:

../lib/route/classid.c: In function 'rtnl_tc_str2handle':
../lib/route/classid.c:187:37: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  187 |                         if (!(colon = strpbrk(str, ":"))) {
      |                                     ^
../lib/route/qdisc/netem.c: In function 'rtnl_netem_set_delay_distribution':
../lib/route/qdisc/netem.c:961:21: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  961 |         test_suffix = strstr(dist_type, dist_suffix);
      |                     ^