community/xfce4-session: add dependency procps

At the Xfce4 shutdown/reboot dialog, option "Save session for future logins" is set by default. This reads out running programs and restores them at the next login. The read-out is done by command ps -p %u -o args= (source). At Alpine Linux, the command ps of busybox is used, that one doesn't offer the option "-p" (reference). The read-out fails, leaving an empty restore command line in file ~/.cache/sessions/xfce4-session-HOSTNAME:0. This leads to Xfce4 session failing to start at the next reboot. I experienced this when installing package calls on postmarketOS, two reboots later Xfce4 was broken (at first reboot the calls daemon was autostarted, at the second reboot it broke).

I filed an issue upstream but don't expect a solution soon: https://gitlab.xfce.org/xfce/xfce4-session/-/issues/128

Therefore let's add package procps as a dependency of xfce4-session. It offers a ps command with the according option "-p" (reference).