Correctly check for HAVE_STAT_BIRTHTIME in configure.ac checks for System.Native...

Correctly check for HAVE_STAT_BIRTHTIME in configure.ac checks for System.Native (mono/mono#16999)

The check was done incorrectly, the include files need to be separated by newlines otherwise we would get a false negative result.

config.log contained this:

```
conftest.c:279:23: warning: extra tokens at end of #include directive [-Wextra-tokens]
#include <sys/types.h>, #include <sys/stat.h>
                      ^
                      //
```

This means that when doing File.GetCreationTime() we'd fall back to returning the last modified time.

It regressed when we switched to System.IO.File to the CoreFX implementation i.e. System.Native in https://github.com/mono/mono/commit/mono/mono@8f5cef936491e8b20888bbf18d426482c890637c.

Fixes https://github.com/mono/mono/issues/16974

Commit migrated from https://github.com/mono/mono/commit/9b98db9ca949bbb706a3d647662985347816d9f7