(Attempt to) fix compile issue in Raytracing

Currently in Manjaro/Arch/Linux there's this issue:
```
/.../src/Mod/Raytracing/App/LuxProject.cpp: In member function ‘virtual App::DocumentObjectExecReturn* Raytracing::LuxProject::execute()’:
/.../src/Mod/Raytracing/App/LuxProject.cpp:85:20: error: variable ‘std::ifstream file’ has initializer but incomplete type
   85 |     ifstream file (fi.filePath().c_str());
      |                    ^~
/.../src/Mod/Raytracing/App/LuxProject.cpp:89:20: error: variable ‘std::ofstream ofile’ has initializer but incomplete type
   89 |     ofstream ofile(tempName.c_str());
      |                    ^~~~~~~~
```

See https://stackoverflow.com/questions/50355130/variable-stdofstream-outfile-has-initializer-but-incomplete-type.

Also edit `Precompiled.h` as compile fix

src/Mod/Raytracing/App/LuxProject.cpp

@@ -24,6 +24,7 @@
 
 #ifndef _PreComp_
 # include <Standard.hxx>
+# include <fstream>
 # include <iosfwd>
 # include <string>
 # include <vector>

src/Mod/Raytracing/App/PreCompiled.h

@@ -28,6 +28,7 @@
 #ifdef _PreComp_
 
 // STL
+#include <fstream>
 #include <iosfwd>
 #include <sstream>
 #include <vector>