CSS: Use i64 in StepsEasingFunction

Unbreaks Windows build. resolve_integer returns an i64 making creation
of a StepsEasingFunction with its result a narrowing conversion before
this change as a long is 32 bits on Windows.

Libraries/LibWeb/CSS/EasingFunction.h

@@ -42,7 +42,7 @@ struct CubicBezierEasingFunction {
 };
 
 struct StepsEasingFunction {
-    long interval_count;
+    i64 interval_count;
     StepPosition position;
     String stringified;