ds4_server: silence the warning

[alantsev]

to silence the warning

ds4_server.c: In function ‘stop_list_find_from’:
ds4_server.c:962:19: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  962 |         char *p = strstr(text + from, stops->v[i]);
      |                   ^~~~~~
In file included from tests/ds4_test.c:3:
tests/../ds4_server.c: In function ‘stop_list_find_from’:
tests/../ds4_server.c:962:19: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  962 |         char *p = strstr(text + from, stops->v[i]);
      |                   ^~~~~~