libstdc++: Fix PSTL test that fails in C++20

This test fails in C++20 and later due to a warning:

warning: C++20 says that these are ambiguous, even though the second is reversed:
note: candidate 1: 'bool MyClass::operator==(const MyClass&)'
note: candidate 2: 'bool MyClass::operator==(const MyClass&)' (reversed)
note: try making the operator a 'const' member function
FAIL: 26_numerics/pstl/numeric_ops/transform_reduce.cc (test for excess errors)

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/pstl/numeric_ops/transform_reduce.cc:
	Add const to equality operator.