[client] x11: don't care if SDL has XInput support anymore

As the X11 code is all self contained now and can be disabled at
configure time there is no longer any need to check if SDL has X11
XInput support.

client/src/main.c

@@ -1930,6 +1930,9 @@ static int lg_run(void)
     return -1;
   }
 
+  // enable WM events
+  SDL_EventState(SDL_SYSWMEVENT, SDL_ENABLE);
+
   g_state.ds->init(&g_state.wminfo);
 
   SDL_SetHint(SDL_HINT_VIDEO_MINIMIZE_ON_FOCUS_LOSS,