fix compilation on Linux

src/gcodeExport.cpp:63:23: error: cannot convert ‘fpos_t {aka _G_fpos_t}’ to ‘long int’ for argument ‘2’ to ‘int fseek(FILE*, long int, int)’
     fseek(f, oldPos, SEEK_SET);
Merge pull request #80 from martinxyz/master

fix compilation on Linux