add str_is_prefix function with static attribute

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@58 bbd45198-f89e-11dd-88c7-29a3b14d5316

finsh/cmd.c

@@ -369,7 +369,7 @@ int list()
 }
 FINSH_FUNCTION_EXPORT(list, list all symbol in system)
 
-int str_is_prefix(const char* prefix, const char* str)
+static int str_is_prefix(const char* prefix, const char* str)
 {
 	while ((*prefix) && (*prefix == *str))
 	{