libstdc++: Fix constant expressions in std::uniform_int_distribution

libstdc++: Fix constant expressions in std::uniform_int_distribution

Clang and EDG say the class member access expressions __urng.min() and
__urng.max() are not constant expressions, because the object expression
__urng is not usable in a constant expresion. Use a qualified-id to call
those static member functions instead.

Co-authored-by: Stephan Bergmann <sbergman@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/uniform_int_dist.h (uniform_int_distribution::_S_nd):
	Use qualified-id to refer to static member functions.