LibWeb: Resolve cyclic declaration/definitions involving Length

This remained undetected for a long time as HeaderCheck is disabled by
default. This commit makes the following file compile again:

    // file: compile_me.cpp
    #include <LibWeb/CSS/GridTrackSize.h>
    // That's it, this was enough to cause a compilation error.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -38,6 +38,7 @@ set(SOURCES
     CSS/GridTrackPlacement.cpp
     CSS/GridTrackSize.cpp
     CSS/Length.cpp
+    CSS/LengthBox.cpp
     CSS/MediaList.cpp
     CSS/MediaQuery.cpp
     CSS/MediaQueryList.cpp