little server browser fix

In the server browser, servers which gametype begin with DM, TDM or CTF (if it's none of those three) won't get the non-pure icon (but they are still filtered out)

So here's my proposition to fix it:
in src/game/client/components/menus_browser.cpp, line 231.
Use the strcmp function instead of strncmp (and then forget the length argument)