AK: Make it possible to not using AK classes into the global namespace

This patch adds the `USING_AK_GLOBALLY` macro which is enabled by
default, but can be overridden by build flags.

This is a step towards integrating Jakt and AK types.

AK/MemoryStream.h

@@ -272,7 +272,9 @@ private:
 
 }
 
+#if USING_AK_GLOBALLY
 using AK::DuplexMemoryStream;
 using AK::InputMemoryStream;
 using AK::InputStream;
 using AK::OutputMemoryStream;
+#endif