AK: Cleanup missing includes and #ifdef evaluation

Problem:
- Several files have missing includes. This results in complaints from
  `clang-tidy`.
- `#ifdef` is followed by `#elif <value>` which evaluates to `0`.

Solution:
- Add missing includes.
- Change to `#elif defined(<value>)`.

AK/Optional.h

@@ -30,6 +30,7 @@
 #include <AK/Platform.h>
 #include <AK/StdLibExtras.h>
 #include <AK/Types.h>
+#include <AK/kmalloc.h>
 
 namespace AK {