astro/gpxloggerd: Unbreak on DragonFly

The latest commits broke this port on DragonFly. The port uses the
base mk files rather than a standalone makefile, and will not build
without warnings appearing:

gpxloggerd.c:185:16: error: variable 'pt' set but not used
                     [-Werror=unused-but-set-variable]

To ensure -Werror flag is not passed to the build, set NO_WERROR for
the makefile.

Approved by: blanket (-Werror, restore DF)