[Shaman] Suppress a Conversion Warning

[Saeldur]

Fixes a type conversion warning caused by it incorrectly using rng().range on double,double assumed types due to the inputs not matching by forcing it to use it for size_t, which then converts implicitly without any precision loss fine.
Shouldn't cause any issues but I haven't confirmed if behaviour is altered (negatively or positively)

[renanthera]

Alternatively, you could replace nearly the whole method with rng().shuffle( ++tl.begin(), tl.end() );, but it certainly has subtly different behaviour to the current implementation, even with the shuffle-thoroughness variable set to 1.0.