When the comparison with a nullptr_t is ill-formed, there is an
additional error for C++11 mode due to the constexpr function body being
invalid.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* testsuite/20_util/tuple/comparison_operators/overloaded2.cc:
	Add dg-error for c++11_only target.

Tested powerpc64le-linux. Committed to trunk.