fixed a compiling error while defining RTT_USING_PPP and using minilibc

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2474 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/libc/minilibc/sys/types.h

@@ -22,4 +22,6 @@ typedef int pid_t;
 #define NULL RT_NULL
 #endif
 
+#define __u_char_defined
+
 #endif