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
```