fix compile error on MDK 3.5 version.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@176 bbd45198-f89e-11dd-88c7-29a3b14d5316

filesystem/dfs/include/dfs_util.h

@@ -28,6 +28,7 @@ int str_is_prefix(const char* prefix, const char* str);
 #if !defined(RT_USING_MINILIBC) && !defined(RT_USING_NEWLIB)
 char *strrchr(const char *t, int c);
 #if defined (__ARMCC_VERSION) && (__ARMCC_VERSION / 10000 < 35)
+#include <stddef.h>
 int strncasecmp(const char* s1, const char* s2, size_t len);
 #endif /* end of __ARMCC_VERSION */
 #endif