fix missing fstream include

[jasjuang]

Otherwise there will be compilation error

/home/jasjuang/SfM-Toy-Library/SfMToyLib/SfM.cpp:636:18: error: variable ‘std::ofstream ofs’ has initializer but incomplete type
     ofstream ofs(prefix + "_points.ply");
                  ^
/home/jasjuang/SfM-Toy-Library/SfMToyLib/SfM.cpp:668:19: error: variable ‘std::ofstream ofsc’ has initializer but incomplete type
     ofstream ofsc(prefix + "_cameras.ply");