Include missing functional header for g++-12

Fixes
```
/opm/input/eclipse/Schedule/Schedule.hpp:241:14: error: ‘function’ in namespace ‘std’ does not name a template type
  241 |         std::function<std::unique_ptr<SegmentMatcher>()> segmentMatcherFactory(std::size_t report_step) const;
      |              ^~~~~~~~
```

opm/input/eclipse/Schedule/Schedule.hpp

@@ -30,6 +30,7 @@
 #include <unordered_map>
 #include <utility>
 #include <vector>
+#include <functional>
 
 #include <opm/input/eclipse/EclipseState/Runspec.hpp>
 #include <opm/input/eclipse/Schedule/Group/Group.hpp>