openat: Work around compilation error with OSF/1 5.1 DTK cc.

openat: Work around compilation error with OSF/1 5.1 DTK cc.

* lib/fopen.c: Use different syntax for include of <stdio.h>.
* lib/freopen.c: Likewise.
* lib/fstatat.c: Use different syntax for include of <sys/stat.h>.
* lib/lstat.c: Likewise.
* lib/stat.c: Likewise.
* lib/open.c: Use different syntax for include of <fcntl.h>.
* lib/openat.c: Include fcntl.h again, explicitly.