[efi] Mark UsbHostController.h as a non-imported header

[efi] Mark UsbHostController.h as a non-imported header

The UsbHostController.h header has been removed from the EDK2 codebase
since it was never defined in a released UEFI specification.  However,
we may still encounter it in the wild and so it is useful to retain
the GUID and the corresponding protocol name for debug messages.

Add an iPXE include guard to this file so that the EDK2 header import
script will no longer attempt to import it from the EDK2 tree.

Signed-off-by: Michael Brown <mcb30@ipxe.org>