file_sys: Move IPS patching code into separate source file

In anticipation of a new BPS patcher.

src/core/file_sys/patch.h

@@ -0,0 +1,15 @@
+// Copyright 2019 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <vector>
+
+#include "common/common_types.h"
+
+namespace FileSys::Patch {
+
+bool ApplyIpsPatch(const std::vector<u8>& patch, std::vector<u8>& buffer);
+
+} // namespace FileSys::Patch