Fix setting random seed to current time

[Vankata453]

Setting the random seed to 0 or lower of a Random generator will now set it to the UNIX timestamp of the current time.

[tobbi]

Users/runner/work/supertux/supertux/src/math/random.cpp:36:22: error: implicit conversion loses integer precision: 'time_t' (aka 'long') to 'std::mersenne_twister_engine<unsigned int, 32, 624, 397, 31, 2567483615, 11, 4294967295, 7, 2636928640, 15, 4022730752, 18, 1812433253>::result_type' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
    m_generator.seed(std::time(nullptr));

[tobbi]

Yeah, although you could've just assigned to v directly without that return.

[mrkubax10]

Yeah, although you could've just assigned to v directly without that return.

True

[Vankata453]

Yeah, although you could've just assigned to v directly without that return.

Indeed, realized that later, but I thought it's still better to directly cast to unsigned int.