fix preprocessor mistake.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2445 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/libc/newlib/syscalls.c

@@ -82,7 +82,7 @@ _link_r(struct _reent *ptr, const char *old, const char *new)
 _off_t
 _lseek_r(struct _reent *ptr, int fd, _off_t pos, int whence)
 {
-#ifndef RT_USING_DfS
+#ifndef RT_USING_DFS
 	return 0;
 #else
 	_off_t rc;