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/Forward.h

@@ -67,6 +67,7 @@ class ImageStyleValue;
 class InheritStyleValue;
 class InitialStyleValue;
 class Length;
+class LengthBox;
 class LengthPercentage;
 class LengthStyleValue;
 class LinearGradientStyleValue;