book: add missing header file (changkun#233)

gliangbupt · web-flow · commit dd7b707acf71 · 2022-07-17T12:42:17.000+02:00
diff --git a/book/en-us/02-usability.md b/book/en-us/02-usability.md
@@ -285,6 +285,8 @@ such as:
 ```cpp
 #include <initializer_list>
 #include <vector>
+#include <iostream>
+
 class MagicFoo {
 public:
     std::vector<int> vec;
diff --git a/book/zh-cn/02-usability.md b/book/zh-cn/02-usability.md
@@ -233,6 +233,8 @@ int main() {
 ```cpp
 #include <initializer_list>
 #include <vector>
+#include <iostream>
+
 class MagicFoo {
 public:
     std::vector<int> vec;
