Wrong Checksum sent for $PGRMZ in GNSSHelper.cpp

[PeteMilligan99]

The carriage return wasn't being checksummed, but the leading dollar sign was. This was providing the wrong checksum to an attached Kobo running XCSoar.

snprintf((char ) &GNSSbuf[GNSS_cnt+1], 24, "\n$PGRMZ,%d,f,3", altitude ); /* feet , 3D fix */
size_t sentence_size = strlen((char *) &GNSSbuf[GNSS_cnt+1]);

        //calculate the checksum
        unsigned char cs = 0;
        for (unsigned int n = 2; n < sentence_size - 1; n++) { **// n=1 changed to n=2**
          cs ^= GNSSbuf[(GNSS_cnt+1) + n];
        }

[lyusupov]

fixed in cee531d