Make dirfd() publically available. It used on other systems as well it seems.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@26466 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/posix/dirent.h

@@ -40,6 +40,9 @@ void			rewinddir(DIR *dir);
 void 			seekdir(DIR *dir, long int loc);
 long int		telldir(DIR *);
 
+/* Non-POSIX extension to get the FD out of the private DIR */
+int				dirfd(DIR *dir);
+
 #ifdef __cplusplus
 }
 #endif