Add <new> header for placement new

Summary: This header is necessary for the placement new operator used in the construct method (line 265)

Reviewed By: yfeldblum

Differential Revision: D3156741

fb-gh-sync-id: 84a64821553b42d46fb70b5ee267a401028c94f2
fbshipit-source-id: 84a64821553b42d46fb70b5ee267a401028c94f2

folly/Optional.h

@@ -54,6 +54,7 @@
  *  }
  */
 #include <cstddef>
+#include <new>
 #include <stdexcept>
 #include <type_traits>
 #include <utility>