Fix includes for M_PI on Win

Fix missing ImportExport

src/Mod/Import/App/dxf.cpp

@@ -4,9 +4,13 @@
 // modified 2018 wandererfan
 
 #include "PreCompiled.h"
-#include <iomanip>
+
+//required by windows for M_PI definition
+#define _USE_MATH_DEFINES
 #include <cmath>
 
+#include <iomanip>
+
 #include <src/Build/Version.h>
 #include <Base/Console.h>
 #include <Base/FileInfo.h>

src/Mod/Import/App/dxf.h

@@ -121,7 +121,7 @@ struct LWPolyDataOut
 };
 //********************
 
-class CDxfWrite{
+class ImportExport CDxfWrite{
 private:
     std::ofstream* m_ofs;
     bool m_fail;

src/Mod/TechDraw/Gui/PreCompiled.h

@@ -33,11 +33,13 @@
 # define PartExport         __declspec(dllimport)
 # define TechDrawGuiExport   __declspec(dllexport)
 # define SpreadsheetExport  __declspec(dllimport)
+# define ImportExport  __declspec(dllimport)
 #else // for Linux
 # define TechDrawExport
 # define PartExport
 # define TechDrawGuiExport
 # define SpreadsheetExport
+# define ImportExport
 #endif
 
 #ifdef _MSC_VER