drivers: fix -Wstringop-truncation

GCC 9 complained about stringop-truncation which is a cautionary message
to prevent using strncpy with non-null terminated strings.

We can fix this by copying one byte less than the destination size and
then manually adding the null termination, as we already do.

src/drivers/gps/gps.cpp

@@ -285,7 +285,7 @@ GPS::GPS(const char *path, gps_driver_mode_t mode, GPSHelper::Interface interfac
 	_instance(instance)
 {
 	/* store port name */
-	strncpy(_port, path, sizeof(_port));
+	strncpy(_port, path, sizeof(_port) - 1);
 	/* enforce null termination */
 	_port[sizeof(_port) - 1] = '\0';