libstdc++: Fix some -Wsign-compare warnings in headers

libstdc++: Fix some -Wsign-compare warnings in headers

In all these cases we know the value with signed type is not negative so
the cast is safe.

libstdc++-v3/ChangeLog:

	* include/bits/deque.tcc (deque::_M_shrink_to_fit): Cast
	difference_type to size_type to avoid -Wsign-compare warning.
	* include/std/spanstream (basic_spanbuf::seekoff): Cast
	streamoff to size_t to avoid -Wsign-compare warning.