graphics/pgplot: fix build on armv7
Same fix as for math/slatec.
Approved by: portmgr (build fix blanket)
MFH: 2025Q3
Event: EuroBSDcon Devsummit 2025
See also: f56f31e07cb64c0b183127733bc6cd6af61efea1
(cherry picked from commit e6c598af85b56623d1cfd1a2f0a340784a263fc7)