Update sort_demo.cpp

added the head file for built-in function rand()

Author: jackeylu

src/sort_demo.cpp

@@ -1,5 +1,5 @@
 
-
+#include <cstdlib>
 #include <iostream>
 #include "sort.h"
 
@@ -114,4 +114,4 @@ int main (int argc, char const *argv[])
 
 
 	return 0;
-}
+}