libstdc++: Simplify test by not using std::log2

libstdc++: Simplify test by not using std::log2

This test uses std::log2 without including <cmath>, but it doesn't need
to use it at all. Just get the number of digits from numeric_limits
instead.

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/random/random_device/entropy.cc: Use
	numeric_limits<unsigned>::digits.

(cherry picked from commit 78fd15fd4a641cfac28ce60bf43a3e1ca309537b)