[RNG] Fixed failures in ranlux_24_48_base and ranlux_24_48 tests of subtract_with_carry and discard_block engines

[egrabovskaya]

Since operator>> takes the engine with template parameters for subtract_with_carry_engine and discard_block_engine classes we have multiple competing definitions in the same namespace when 2 or more of these engine types are instantiated. Therefore, it's necessary to move the definition outside of the class, then it's only defined once, and add its declaration before the class definition.

[timmiesmith]

Thank you for the quick work on this issue!