Fix build on s390

This should fix the s390 built on Jenkins that currently fails with:

```
  CC     libmini_la-tramp-s390x.lo
  CC     libmini_la-mini-posix.lo
mini-posix.c: In function ‘mono_runtime_setup_stat_profiler’:
mini-posix.c:686: error: ‘profiling_signal_in_use’ undeclared (first use in this function)
mini-posix.c:686: error: (Each undeclared identifier is reported only once
mini-posix.c:686: error: for each function it appears in.)
make[4]: *** [libmini_la-mini-posix.lo] Error 1
```

Note: I know next to nothing about that code, so if moving the variable declaration causes problems, please let me know :smile: