Rename fpos_t to fpos1 in SdBaseFile.h and SdBaseFile.cpp

[Phraust]

Firmware will not compile in ATOM+PlatformIO otherwise.

According to:
http://forums.reprap.org/read.php?146,691608,699987#msg-699987

"Problem is in C++ you can not use typedef and struct for the same varibale name. fpos_t is already defined in stdio.h which is a hardware library file for arduino."