Declare json in OrcaSlicer.cpp on Every Platform

OrcaSlicer.cpp had its "using namespace nlohmann;" and the json include
inside the Linux-only include block, so on Windows and macOS it took
json from AppConfig.hpp's global directive, which is gone. The include
and a "using json = nlohmann::json;" now sit outside the block.

src/OrcaSlicer.cpp

@@ -77,12 +77,10 @@
 #include <condition_variable>
 #include <mutex>
 #include <boost/thread.hpp>
-//add json logic
-#include "nlohmann/json.hpp"
-
-using namespace nlohmann;
 #endif
 
+#include "nlohmann/json.hpp"
+
 #include <boost/algorithm/string/predicate.hpp>
 #include <boost/filesystem.hpp>
 #include <boost/nowide/args.hpp>
@@ -156,6 +154,7 @@ using namespace nlohmann;
 #include <stdio.h>
 
 namespace fs = boost::filesystem;
+using json = nlohmann::json;
 
 #ifdef __WXGTK__
 #if __has_include(<X11/Xlib.h>)