add missing header file <cstring> for exercise 22

Author: DongDongBan

exercises/22_class_template/main.cpp

@@ -1,5 +1,5 @@
 #include "../exercise.h"
-
+#include <cstring>
 // READ: 类模板 <https://zh.cppreference.com/w/cpp/language/class_template>
 
 template<class T>