Added missing namespace qualification for Slic3r::GUI::format in Http.cpp
Added missing namespace qualification for Slic3r::GUI::format in Http.cpp

prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r