kernel: Remove unnecessary includes

Over the course of the changes to the kernel code, a few includes are no
longer necessary, particularly with the change over to std::shared_ptr
from Boost's intrusive_ptr.

src/core/hle/kernel/writable_event.h

@@ -4,6 +4,8 @@
 
 #pragma once
 
+#include <memory>
+
 #include "core/hle/kernel/object.h"
 
 namespace Kernel {