utils/fs: use <sys/stat.h>, not <sys\stat.h>
utils/fs: use <sys/stat.h>, not <sys\stat.h>

Fix cross-compilation failure from linux to windows:
```
  CC utils/unlit/dist-install/build/fs.o
utils/unlit/fs.c:24:10: fatal error: sys\stat.h: No such file or directory
 #include <sys\stat.h>
          ^~~~~~~~~~~~
```

Signed-off-by: Sergei Trofimovich <110fdadc76b6bce16977f568bdb4723eaed6fb3f@gentoo.org>

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc