This test fails in C++20 mode because std::is_clock is false for the
test clock, because it doesn't define a duration member.

libstdc++-v3/ChangeLog:

	* testsuite/30_threads/condition_variable/members/68519.cc:
	Define recent_epoch_float_clock::duration to meet the Cpp17Clock
	requirements.

Tested powerpc64le-linux. Committed to trunk.