Bug 1671641 - Make `dom/flex` buildable outside of `unified-build` environment. r=sg

Differential Revision: https://phabricator.services.mozilla.com/D94933

build/non-unified-compat

@@ -21,6 +21,7 @@ dom/fetch/
 dom/file/
 dom/filehandle/
 dom/filesystem/
+dom/flex/
 dom/indexedDB/
 dom/localstorage/
 dom/quota/

dom/flex/FlexLineValues.cpp

@@ -6,6 +6,7 @@
 
 #include "FlexLineValues.h"
 
+#include "Flex.h"
 #include "FlexItemValues.h"
 #include "mozilla/dom/FlexBinding.h"
 #include "nsFlexContainerFrame.h"