Remove NXXM. Execute IWYU. (#397)

examples/dom/vflow.cpp

@@ -1,6 +1,7 @@
 #include <stdio.h>                 // for getchar
 #include <ftxui/dom/elements.hpp>  // for operator|, Element, size, text, hcenter, Fit, vflow, window, EQUAL, bold, border, dim, HEIGHT, WIDTH
 #include <ftxui/screen/screen.hpp>  // for Full, Screen
+#include <memory>                   // for shared_ptr
 #include <string>  // for allocator, operator+, to_string, char_traits, string
 
 #include "ftxui/dom/node.hpp"      // for Render