efi_loader: avoid using HandleProtocol in helloworld.efi

HandleProtocol() is deprecated and leaves an OpenedProtocolInformation
behind. Use OpenProtocol(GET_PROTOCOL) instead.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>