Draft, Import, area: include iosfwd instead of iostream if forward declarations are sufficient

src/Mod/Import/App/dxf.cpp

@@ -10,6 +10,9 @@
 #include <cmath>
 
 #include <iomanip>
+#include <fstream>
+#include <sstream>
+#include <iostream>
 
 #include <App/Application.h>
 #include <Base/Console.h>

src/Mod/Import/App/dxf.h

@@ -13,12 +13,10 @@
 #include <vector>
 #include <map>
 #include <set>
-#include <fstream>
-#include <sstream>
-#include <iostream>
-#include <stdio.h>
-#include <string.h>
-#include <math.h>
+#include <iosfwd>
+#include <cstdio>
+#include <cstring>
+#include <cmath>
 
 #include <Base/Vector3D.h>