finance/gnucash: Fix build with ICU 76+
Backport upstream commit 579eed1facc0f7834ea70b1a342ebca0f125d788 to fix
build
Reference:
https://github.com/Gnucash/gnucash/commit/579eed1facc0f7834ea70b1a342ebca0f125d788
PR: 278420
Approved by: portmgr (blanket, build fix)