Fix CMAKE_UNITY_BUILD usage (#738)

Add missing include guard.
This fix https://github.com/cpp-best-practices/cmake_template/issues/64

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/screen/util.hpp

@@ -1,6 +1,9 @@
 // Copyright 2022 Arthur Sonzogni. All rights reserved.
 // Use of this source code is governed by the MIT license that can be found in
 // the LICENSE file.
+#ifndef FTXUI_SCREEN_UTIL_HPP
+#define FTXUI_SCREEN_UTIL_HPP
+
 namespace ftxui {
 namespace util {
 
@@ -12,3 +15,5 @@ constexpr const T& clamp(const T& v, const T& lo, const T& hi) {
 
 }  // namespace util
 }  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_SCREEN_UTIL_HPP */