main/postmarketos-ui-xfce4: add package "procps"

At Xfce4, when installing "Calls" (phone application), it breakes Xfce4 after two reboots (after the first reboot it gets autostarted, at the second reboot it breaks).

Background: By default, the option "Save session for future logins" is set at the shutdown/reboot dialog. This reads out running programs and restores them at the next login. As Calls starts a daemon, this applies to Calls as well. The read-out is done by command "ps -p %u -o args=" (source). At Alpine/pmOS, the command "ps" of busybox is used, that one doesn't offer the option "-p". The read-out fails, leaving an empty restore command line in file ~/.cache/sessions/xfce4-session-HOSTNAME:0.

Package procps (package reference) contains a proper command "ps" offering all options. This solves the problem, "Calls" can be installed without issues.