Use std namespace for div function

Although GCC 4.6.3 implements div by including stdlib.h (where
everything is put in the global namespace), and then importing these
into std, not every compiler does this (one could for instance think
that in #include <stdlib.h> inside the namespace).

opm/core/io/eclipse/EclipseWriter.cpp

@@ -199,7 +199,7 @@ private:
         assert(stride > 0 && stride < num - offset);
 
         // number of (strided) entries it will provide
-        const div_t d = div (data.size () - offset, stride);
+        const div_t d = std::div (data.size () - offset, stride);
         return d.quot;
     }
 };