libstdc++: Change class-key for duration and time_point to class

We define these with the 'struct' keyword, but the standard uses
'class'. This results in warnings if users try to refer to them using
elaborated type specifiers.

libstdc++-v3/ChangeLog:

	* include/bits/chrono.h (duration, time_point): Change 'struct'
	to 'class'.