LibWeb: 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 <LibWeb/HTML/CrossOrigin/CrossOriginOpenerPolicy.h>
    // That's it, this was enough to cause a compilation error.

Likewise for most other files touched by this commit.

Userland/Libraries/LibWeb/HTML/Parser/HTMLEncodingDetection.h

@@ -8,6 +8,7 @@
 
 #include <AK/Optional.h>
 #include <AK/String.h>
+#include <LibJS/Heap/GCPtr.h>
 #include <LibWeb/Forward.h>
 
 namespace Web::HTML {