fix typo in guard macro of fsh.h

on CLang 3.4 svn this produced

```
/home/erne/src/opm-core/opm/core/pressure/fsh.h:20:9: warning: 'OPM_FSH_HEADER_INCLUDED' is used as a
      header guard here, followed by #define of a different macro [-Wheader-guard]
```

which is correct...