sysutils/plasma5-ksysguard: reinstate PORTREVISION

sysutils/plasma5-ksysguard is locked at 5.22.0 as it is no longer
released as part of KDE Plasma. The upgraded of the rest of the KDE
Plasma Desktop to 5.24.4 in [1] removed its PORTREVISION therefore
wrongly.

[1] https://cgit.freebsd.org/ports/commit/?id=e52cbf14ed550f8f8027405de9609878135c8127

Reported by: erwin