security/openconnect-gui: fix fetch and bump PORTREVISION
I recalculated distinfo myself using `make makesum`, so it differs from
the distinfo given in the patch.
Note that version 1.5.3 is released for those who want to pick up this port.
PR: 245244
Submitted by: papowell@astart.com
Approved by: maintainer (pkbubaj)
MFH: 2020Q2