libstdc++: Add nodiscard to std::find

I missed this one out in r14-9478-gdf483ebd24689a but I don't think that
was intentional. I see no reason std::find shouldn't be [[nodiscard]].

libstdc++-v3/ChangeLog:

	* include/bits/stl_algo.h (find): Add nodiscard.

Reviewed-by: Patrick Palka <ppalka@redhat.com>