math/csdp: add missing include directive

gcc treats an implicit declaration as an error these days. Shut it up
by supplying the declaration of printf() through <stdio.h>.

Approved by: portmgr (build fix blanket)
MFH: 2026Q2

(cherry picked from commit 8fb9ba4edf44eecb887014525e64d080c8e2ef5e)