Remove using namespace std from json_diff.hpp

The directive sat at global scope in a header that DeviceManager.hpp
includes, so most of the GUI compiled with all of std in the global
namespace. 42 files had come to rely on it, mostly for string, vector
and unordered_map, four of them for the ""sv and ""ms literals.

Those sites are qualified. GCodeViewer.cpp spelled the type as
std::vector<::string>, which only resolved through the directive.

src/slic3r/GUI/DeviceCore/DevFilaSystem.cpp

@@ -161,7 +161,7 @@ DevAms::~DevAms()
     m_trays.clear();
 }
 
-static unordered_map<int, wxString> s_ams_display_formats = {
+static std::unordered_map<int, wxString> s_ams_display_formats = {
     {DevAms::AMS,      "AMS-%d"},
     {DevAms::AMS_LITE, "AMS Lite-%d"},
     {DevAms::N3F,      "AMS 2 PRO-%d"},