Platform/Socionext/DeveloperBox: Fix build break due to need for RngLib

Commit b5701a4c7a0f ("CryptoPkg: OpensslLib: Use RngLib to generate
entropy in rand_pool") introduced RngLib dependency for OpensslLib
and caused Developerbox platforms build to break for lack of
an RngLib instance.

Add the reference to RngLib to the DeveloperBox.dsc.inc file.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>