fix export of base class

src/Mod/Import/App/dxf.cpp

@@ -3,6 +3,7 @@
 // This program is released under the BSD license. See the file COPYING for details.
 // modified 2018 wandererfan
 
+#include "PreCompiled.h"
 #include "dxf.h"
 
 using namespace std;

src/Mod/Import/App/dxf.h

@@ -138,7 +138,7 @@ public:
 };
 
 // derive a class from this and implement it's virtual functions
-class CDxfRead{
+class ImportExport CDxfRead{
 private:
     std::ifstream* m_ifs;