memory_hook: Default virtual destructor in the cpp file

Prevents creating multiple copies of the vtable in every translation unit that uses the class.
Also silences a -Wweak-vtables warning

src/core/memory_hook.cpp

@@ -0,0 +1,11 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include "core/memory_hook.h"
+
+namespace Memory {
+
+MemoryHook::~MemoryHook() = default;
+
+} // namespace Memory