graphics/gimp-app: fpsetmask() -> fedisableexcept()

Replace the use of the older fpsetmask() call with fedisableexcept().
The build previously only succeeded on armv7 due to ANSI C semantics
in which it is permitted to call an undlecared function.

Approved by: portmgr (build fix blanket)
MFH: 2025Q4