Include <config.h> to fix build on GCC 4.8

GCC 4.8 provides 'nullptr' in C++11 mode.  That capability is
detected at configuration time and stored in <config.h> and we need
it when targeting Dune 2.2.1 to avoid diagnostics of the form

  [...]/dune/common/nullptr.hh:27:1:
  error: expected ‘;’ after class definition
   } nullptr = {};              // and whose name is nullptr
    ^

  [...]/dune/common/nullptr.hh:27:1:
  error: qualifiers can only be specified for objects and functions
  [...]/dune/common/nullptr.hh:27:3:
  error: expected unqualified-id before ‘nullptr’
   } nullptr = {};              // and whose name is nullptr

examples/opm_init_check.cpp

@@ -17,6 +17,7 @@
   along with OPM.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include <config.h>
 
 #include <iostream>
 #include <fstream>