Libraries: Add missing includes, add namespace qualifiers

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

Likewise for most other files touched by this commit.

Userland/Libraries/LibGPU/StencilConfiguration.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <LibGPU/Config.h>
 #include <LibGPU/Enums.h>
 
 namespace GPU {