MdePkg/BaseRngLibDxe: Add DXE_RUNTIME_DRIVER class to RngLib

MdePkg/BaseRngLibDxe: Add DXE_RUNTIME_DRIVER class to RngLib

The Raspberry Pi platform with Secure Boot enabled currently fails to build
with error:

  Module type [DXE_RUNTIME_DRIVER] is not supported by library instance
  [/home/appveyor/projects/rpi4/edk2/MdePkg/Library/DxeRngLib/DxeRngLib.inf]

Add the missing class to fix this issue.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Samer El-Haj-Mahmoud <Samer.El-Haj-Mahmoud@arm.com>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>