x11-wm/xfce4-session: Add runtime depend on xrdb

The included xinitrc script uses the xrdb program.

It is installed by default by the full xorg metaport, but not by
the minimal version.

PR: 265165
MFH: 2022Q3
(cherry picked from commit 2c25ab61821fe05f1ff529a503b4cae4d145e57e)