LibWeb: Add missing headers

Userland/Libraries/LibWeb/CSS/Display.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+#include <AK/Assertions.h>
+
 namespace Web::CSS {
 
 class Display {

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <LibWeb/Layout/Box.h>
 #include <LibWeb/Layout/FormattingContext.h>
 
 namespace Web::Layout {

Userland/Libraries/LibWeb/Layout/FormattingContext.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/OwnPtr.h>
 #include <LibWeb/Forward.h>
 
 namespace Web::Layout {

Userland/Libraries/LibWeb/Layout/TreeBuilder.h

@@ -8,6 +8,7 @@
 
 #include <AK/NonnullRefPtrVector.h>
 #include <AK/RefPtr.h>
+#include <LibWeb/CSS/Display.h>
 #include <LibWeb/Forward.h>
 
 namespace Web::Layout {