Explicitly cast the return value

When this library was used with bios-bmc-smm-error-logger, it was
failing because of a clang-tidy warning(For some reason clang-tidy
runs on subprojects/libbej as well.).

We need to explicitly cast to stop this warning.

Tested:
Tested running CI locally for bios-bmc-smm-error-logger with this
change and it passed.

Signed-off-by: Kasun Athukorala <kasunath@google.com>
Change-Id: Ia8a33a08576cc0f2de50ecba77f3b57eab7fe48e