I don't think this is actually required to compile, because using
operator<< without a definition of the ostream doesn't seem valid to me.
But it's easy to make it work.

	PR libstdc++/94051
	* include/std/string_view: Include <bits/ostream_insert.h>.
	* testsuite/21_strings/basic_string_view/inserters/94051.cc: New test.