[cleanup] include header for std::function

std::function is defined in <functional>. Prior to GCC 7 this header was
transitively included through other headers in the C++ Standard Library
but in GCC 7 these transitive includes have been removed.

See https://gcc.gnu.org/gcc-7/porting_to.html#header-dep-changes

dune/common/parameterizedobject.hh

@@ -3,6 +3,7 @@
 #ifndef DUNE_COMMON_PARAMETERIZEDOBJECT_HH
 #define DUNE_COMMON_PARAMETERIZEDOBJECT_HH
 
+#include <functional>
 #include <map>
 
 #include <dune/common/exceptions.hh>