Externals: Add zstd

I had to rename Source/Common/Compiler.h because the VS build
confuses it with Externals/zstd/lib/common/compiler.h otherwise.

Source/Core/Common/Inline.h

@@ -0,0 +1,11 @@
+// Copyright 2018 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#ifdef _WIN32
+#define DOLPHIN_FORCE_INLINE __forceinline
+#else
+#define DOLPHIN_FORCE_INLINE inline __attribute__((always_inline))
+#endif