games/wesnoth: fix build following addition of unreachable(3)

This avoids a name collision between the C23 unreachable() macro from
<stddef.h> and a local variable, fixing the build on FreeBSD 16-CURRENT.

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

(cherry picked from commit f2611f144b2515efc3e372385a6439fad0271908)