astro/gpsbabel: unbreak with clang 4.0

bushnell.cc:138:36: error: ordered comparison between pointer and zero ('const char *' and 'int')

for (t = bushnell_icons; t->icon > 0; t++) {
                         ~~~~~~~ ^ ~

bushnell.cc:150:36: error: ordered comparison between pointer and zero ('const char *' and 'int')

for (t = bushnell_icons; t->icon > 0; t++) {
                         ~~~~~~~ ^ ~

PR: 216215
Submitted by: Oliver Heesakkers <dev2@heesakkers.info> (maintainer)