[patch] libstdc++/65499 make duration literals visible in std::chrono namespace

[Jonathan Wakely]

This is a tiny tweak to add a missing piece of C++14 support, the
duration literals should be usable via "using namespace std::chrono".
It doesn't affect anything in C++03 or C++11 mode so safe for trunk
now.

Tested x86_64-linux, committed to trunk.

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