Missing gps_ prefix

[EtienneAr]

Just a little mistake here : the "gps_" prefix is missing before "encode(char c)" in tinygps.h . (in tinygps.c the function is gps_encode(char c), which is consistent with the other functions).

https://github.com/acassis/tinygps/blob/64e29d9158f318ff8deae1d94a680e5afbf57854/tinygps.h#L65