Added include of qnan.h (now that AssimpPCH.h include was taken out)

code/PlyExporter.cpp

@@ -48,7 +48,8 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include "../include/assimp/scene.h"
 #include "../include/assimp/version.h"
 #include "../include/assimp/IOSystem.hpp"
-#include "../include/assimp/Exporter.hpp"
+#include "../include/assimp/Exporter.hpp"
+#include "qnan.h"
 
 
 using namespace Assimp;