Kernel: Decorate KResultOr with [[nodiscard]] to catch misbehaving callers

The [[nodiscard]] decorator enables us to have the compile emit a
warning anytime the decorated type or function's return value isn't
observed. It's very useful for catching error checking bugs in systems
which use C style error handling.

Kernel/KResult.h

@@ -61,7 +61,7 @@ private:
 };
 
 template<typename T>
-class alignas(T) KResultOr {
+class alignas(T) [[nodiscard]] KResultOr {
 public:
     KResultOr(KResult error)
         : m_error(error)