Fix incomplete type error for std::ofstream in examples

On my system I got
```c++
error: variable ‘std::ofstream file’ has initializer but incomplete type
         std::ofstream file(fname.str().c_str());
```
This is fixed with this commit by including fstream. Previously,
this include might have happened implicitely.

examples/compute_initial_state.cpp

@@ -36,6 +36,8 @@
 
 #include <boost/filesystem.hpp>
 
+#include <fstream>
+
 namespace
 {
     void warnIfUnusedParams(const Opm::parameter::ParameterGroup& param)