Add a _LIBCPP_CONSTEXPR that was missed in r170026.

(clang doesn't complain about this, but gcc does.  This is necessary for a
follow-up patch that will enable _LIBCPP_CONSTEXPR for gcc.)


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@209888 91177308-0d34-0410-b5e6-96231b3b80d8