x11/xfce4-screensaver: Use pgrep instead of pidof

Import patch accepted upstream to use pgrep, instead of pidof, which
is not a dependency of this port.

Use switching from the screen saver should be fixed by this change.

PR: 290766
Obtained from: https://gitlab.xfce.org/apps/xfce4-screensaver/-/merge_requests/58