HAL_SITL: fixed warning building with gcc 9.x

libraries/AP_HAL_SITL/SITL_State.h

@@ -151,7 +151,7 @@ private:
     void _update_gps_mtk(const struct gps_data *d, uint8_t instance);
     void _update_gps_mtk16(const struct gps_data *d, uint8_t instance);
     void _update_gps_mtk19(const struct gps_data *d, uint8_t instance);
-    uint16_t _gps_nmea_checksum(const char *s);
+    uint8_t _gps_nmea_checksum(const char *s);
     void _gps_nmea_printf(uint8_t instance, const char *fmt, ...);
     void _update_gps_nmea(const struct gps_data *d, uint8_t instance);
     void _sbp_send_message(uint16_t msg_type, uint16_t sender_id, uint8_t len, uint8_t *payload, uint8_t instance);

libraries/AP_HAL_SITL/sitl_gps.cpp

@@ -649,9 +649,9 @@ void SITL_State::_update_gps_mtk19(const struct gps_data *d, uint8_t instance)
 /*
   NMEA checksum
  */
-uint16_t SITL_State::_gps_nmea_checksum(const char *s)
+uint8_t SITL_State::_gps_nmea_checksum(const char *s)
 {
-    uint16_t cs = 0;
+    uint8_t cs = 0;
     const uint8_t *b = (const uint8_t *)s;
     for (uint16_t i=1; s[i]; i++) {
         cs ^= b[i];
@@ -665,7 +665,7 @@ uint16_t SITL_State::_gps_nmea_checksum(const char *s)
 void SITL_State::_gps_nmea_printf(uint8_t instance, const char *fmt, ...)
 {
     char *s = nullptr;
-    uint16_t csum;
+    uint8_t csum;
     char trailer[6];
 
     va_list ap;