WindowServer: Add missing includes

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 <WindowServer/SystemEffects.h>
    // That's it, this was enough to cause a compilation error.

Userland/Services/WindowServer/SystemEffects.h

@@ -6,7 +6,9 @@
 
 #pragma once
 
+#include <AK/StringView.h>
 #include <AK/Types.h>
+#include <AK/Vector.h>
 
 namespace WindowServer {