fix discards const from pointer target

[heitbaum]

Since glibc-2.43:

For ISO C23, the functions bsearch, memchr, strchr, strpbrk, strrchr, strstr, wcschr, wcspbrk, wcsrchr, wcsstr and wmemchr that return pointers into their input arrays now have definitions as macros that return a pointer to a const-qualified type when the input argument is a pointer to a const-qualified type.

https://lists.gnu.org/archive/html/info-gnu/2026-01/msg00005.html

../../alsactl/init_parse.c: In function 'elemid_get':
../../alsactl/init_parse.c:695:29: warning: initialization discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  695 |                 char *pos = strchr(attr, ' ');
      |                             ^~~~~~
../../alsactl/init_parse.c: In function 'conf_name_filter':
../../alsactl/init_parse.c:1284:21: warning: initialization discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1284 |         char *ext = strrchr(d->d_name, '.');
      |                     ^~~~~~~
../../aplay/aplay.c: In function 'create_path':
../../aplay/aplay.c:3226:23: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 3226 |                 start = strchr(path + 1, '/');
      |                       ^
../../aplay/aplay.c:3228:23: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 3228 |                 start = strchr(path, '/');
      |                       ^
../../topology/pre-process-object.c: In function 'tplg_pp_add_object_tuple_section':
../../topology/pre-process-object.c:562:14: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  562 |         type = strchr(token_ref, '.');
      |              ^
../../topology/pre-process-dapm.c:175:14: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  175 |         args = strchr(string, '.');
      |              ^