Add missing include

Summary:
String.h references std::vector, but it is not included.
Closes https://github.com/facebook/folly/pull/496

Reviewed By: yfeldblum, meyering

Differential Revision: D4009410

Pulled By: Orvid

fbshipit-source-id: f21ce0d5833f0959a83e75e2fbcbe2beedbb5b6f

folly/String.h

@@ -20,6 +20,7 @@
 #include <exception>
 #include <stdarg.h>
 #include <string>
+#include <vector>
 #include <boost/type_traits.hpp>
 #include <boost/regex/pending/unicode_iterator.hpp>