Made header includable from C++.

opm/core/transport/reorder/twophasetransport.h

@@ -3,6 +3,10 @@
 #ifndef TWOPHASETRANSPORT_H_INCLUDED
 #define TWOPHASETRANSPORT_H_INCLUDED
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct UnstructuredGrid;
 void twophasetransport(
     const double *porevolume,
@@ -13,4 +17,8 @@ void twophasetransport(
     const int *satnum,
     double *saturation);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* TWOPHASETRANSPORT_H_INCLUDED */