libstdc++: Fix test that uses C++17 variable template in C++14

libstdc++: Fix test that uses C++17 variable template in C++14

This test fails if run with -std=gnu++14 because it should be using
is_convertible instead of is_convertible_v.

libstdc++-v3/ChangeLog:

	* testsuite/experimental/propagate_const/observers/107525.cc:
	Use type trait instead of C++17 variable template.