x11-toolkits/gtk-sharp*: Unbreak ports for lang/mono6.8
See also: https://svnweb.freebsd.org/changeset/ports/558398
PR: 272950
Approved by: portmgr (build fix blanket)
MFH: 2023Q3
(cherry picked from commit 90301b32a60b21fa3b832f079c53177ef3418284)