misc/raspberrypi-gpioshutdown: fix build on recent FreeBSD

Due to -Werror being enabled for kernel modules these days, the build
of this module failed with a "variable set but not used" warning.
Remove the offending variable to fix the build.

Approved by: portmgr (build fix blanket)
MFH: 2024Q4

(cherry picked from commit ed4dd3bfc4326cd2262f5215491eab8cd172a2c2)