efi_loader: add missing EFI_CALL when closing a file

Closing the files uses the EFI protocol and specifically it's .close
callback.  This needs to be wrapped on an EFI_CALL()

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