[build] Include string in v8.h

Explicitly #include<string> in v8.h, since std::string is referenced
in it. In the C++ STL shipped with Visual Studio 2019, none of the
headers included in v8.h ends up including the C++ string header, which
caused a compile error.

Bug: v8:9793
Change-Id: I84a133dd10dd6dcc7b70287af393e82cf0dc97df
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1834321Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64074}

include/v8.h

@@ -19,6 +19,7 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <memory>
+#include <string>
 #include <type_traits>
 #include <utility>
 #include <vector>