Build with -Wmissing-declarations (#71)

This flag is used to find global functions defined without a previous
declaration. Usually it spots accidental globals, but in this case it
was just missing headers.

src/ftxui/dom/separator.cpp

@@ -1,3 +1,4 @@
+#include "ftxui/dom/elements.hpp"
 #include "ftxui/dom/node.hpp"
 
 namespace ftxui {

src/ftxui/dom/text.cpp

@@ -1,4 +1,5 @@
 #include <algorithm>
+#include "ftxui/dom/elements.hpp"
 #include "ftxui/dom/node.hpp"
 #include "ftxui/screen/string.hpp"