d/p/gpsmon-gpsmon.c-Fix-silly-warning.patch: fix FTFBS (Closes: #997178)

Ubuntu had the same FTBFS https://launchpadlibrarian.net/569860174/buildlog_ubuntu-jammy-arm64.gpsd_3.22-4build1_BUILDING.txt.gz

Here is a build log with the fix applied gpsd_3.22-4ubuntu1_amd64.build