fix -Wunused-[...] compiler warnings

xdpyinfo.c:148:1: warning: ‘silent_errors’ defined but not used [-Wunused-function]
  148 | silent_errors(_X_UNUSED Display *dpy, _X_UNUSED XErrorEvent *ev)
xdpyinfo.c:155:14: warning: ‘old_handler’ defined but not used [-Wunused-variable]
  155 | static int (*old_handler)(Display *, XErrorEvent *) = NULL;

Signed-off-by: Thomas Kuehne <thomas@kuehne.cn>
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