graphics/gimp-app: Fix build with librsvg2-rust 2.58.90+

Fix suggested by mikael@ and adapted by fluffy@

Reference:
https://gitlab.gnome.org/GNOME/librsvg/-/blob/2.58.91/NEWS?ref_type=tags

Patch sent privately via Matrix

PR: 284977
Tested by: fluffy
Approved by: portmgr (blanket, build fix)